• 32 cats had unilateral thyroidectomy and 53 had bilateral thyroidectomy, based on a thyroid scan.
• All cats had serum thyroid hormone concentrations at normal or below the normal reference interval within 48 hours of surgery.
• Most cats with bilateral thyroidectomy were treated postoperatively with oral thyroxine supplementation. This was discontinued in most cats within 3–6 months with no evidence of clinical or laboratory hypothyroidism.
• Relapse of hyperthyroidism occurred in four cats, 8–44 months following bilateral thyroidectomy, confirmed with a repeated thyroid imaging.
• Most cats with unilateral thyroidectomy had T4 levels below normal range for 2–3 months after surgery but did not require thyroid supplementation.

• Cats with euthyroidism or hypothyroidism were not separated in analysis.
• Subclinical hypothyroidism not determined using thyroid stimulating hormone (TSH) measurements.
• Only cats with clinical disease were followed-up with long-term sampling.
• Cats did not receive a wash out period for antithyroid drugsprior to surgery.
• Referral population.

• Serum thyroxine (T4) concentration.
• Complete blood count (CBC), biochemistry, urinalysis.
• Glomerular filtration rate (GFR).

• 30 days post-surgery, mean serum T4 was reduced by 90%.
• T4 fell to with the reference interval (10–48 nmol/L) in 6/13 (46%) hyperthyroid cats 30 days after surgery.
• T4 fell below the reference interval in 7/13 (54%) hyperthyroid cats 30 days after surgery.
• All cats had a resolution of clinical signs and gained weight 30 days after bilateral thyroidectomy.
• No significant difference in T4 was found in control cats at day 0 and day 30.

• Relatively small sample sizes in each group.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone (TSH) measurements).
• Follow-up was only at 30 days and no longer term T4 evaluation was performed.

• Resolution or recurrence of clinical signs.
• Plasma T4 levels if measured.

• 95 cats received their first thyroidectomy surgeries at the Utrecht University Clinic by the same surgeon.
• 81 cats received bilateral thyroidectomy with all cats experiencing remission of their hyperthyroidism.
• Five cats had bilateral thyroidectomy and removal of additional ectopic tissue. 57/95 (60%) cats experienced remission at follow-up and 38/95 (40%) had recurrence of disease.
• Recurrence of hyperthyroidism after surgery was significantly higher in cats with ectopic hyperplastic thyroid tissue.
• Surgeon experience may be an important factor in determining the long-term outcome of thyroidectomy, especially with regards to postoperative hypocalcaemia.
• Nine cats had unilateral thyroidectomy and all had remission of disease at follow-up.

• Referral population.
• Follow-up checks were through interview and although some were with referral veterinarians, the follow-up data may be more subjective than objective.
• Remission was defined by interview mainly by resolution of clinical signs. T4 levels were only measured where available. The possibility of cats with subclinical disease cannot be excluded.
• Retrospective studies are relatively low down on the evidence hierarchy.

Longitudinal study (n = 23).

• Bodyweight.
• Total thyroxine (TT4) concentration:
  • Hypothyroidism assessed as TT4 < 10 nmol/L.
  • Euthyroidism assessed as TT4 10–55 nmol/L.
  • Hyperthyroidism assessed as TT4 > 55 nmol/L.
• Thyroid stimulating hormone (TSH) concentration.
• Urine specific gravity (USG).
• Creatinine concentration.
• Symmetric dimethylarginine (SDMA) concentration.

Subjective assessment of:

• Body condition score (BCS).

• 13/68 (19%) cats were euthyroid 6 months post- surgery.
• 15/68 (22%) cats had persistent or recurrent hyperthyroidism 6 months post-surgery.
• 33/68 (49%) cats were hypothyroid 6 months post- surgery.
• Overt hypothyroidism 6 months post-surgery was greater in cats that had both thyroid glands removed in one procedure 46/68 (67.9%) compared to having staged thyroidectomies 27/68 (40%).

Longitudinal study 595–1955 days post-surgery (n = 23):

• 4/23 (17%) cats remained hypothyroid.
• 19/23 (83%) cats were euthyroid transiently.
• 10/23 (44%) cats developed recurrent hyperthyroidism.
• Cats have changes in thyroid function long-term following bilateral thyroidectomy with a high incidence of recurrent hyperthyroidism.

Main findings:

• A high proportion of cats develop recurrent hyperthyroidism following bilateral thyroidectomy.
• Long-term after surgery, cats remain euthyroid, or progress to hyperthyroidism over years.

• Retrospective studies are relatively low down on the evidence hierarchy.
• Samples used had been in storage for many years (72/151 samples stored for 10 years or more at 80 °C) with numerous freeze-thaw cycles.
• Total T4 was only followed-up longer term on a small proportion of cats.
• Medical stabilisation was attempted on 60/68 (88%) cats prior to surgery with varying success.
• Total T4 was only measured in 45 cats prior to surgery showing that only 24 cats had controlled thyroxine (T4) levels prior to surgery.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• Referral population.

• 27 cats were treated with radioiodine (2.1–7.2 mCi, equivalent to 78–267 MBq).
• 22 cats received a bilateral thyroidectomy.
• Nine cats were medically managed with methimazole.

• Serum T4 concentration.
• Renal parameters – serum creatinine, blood urea nitrogen (BUN), urine specific gravity (USG).

• Pretreatment T4 = 11.0 ug/dl (141.6 nmol/L).
• Post-treatment T4 30 days = 1.6 ug/dl (20.6 nmol/L).
• Post-treatment T4 90 days = 2.0 ug/dl (25.7 nmol/L).
• Serum T4 concentrations were significantly lower following radioiodine treatment (P < 0.001) but there was no significant difference between day 30 and day 90 post-treatment.

Bilateral thyroidectomy group (n = 22):

• Pretreatment T4 = 9.3 ug/dl (119.7 nmol/L).
• Post-treatment T4 30 days = 2.6 ug/dl (33.5 nmol/L).
• Post-treatment T4 90 days = 2.2 ug/dl (28.3 nmol/L).
• Serum T4 concentrations were significantly lower following bilateral thyroidectomy (P < 0.001) but there was no significant difference between day 30 and day 90 post-treatment.

No significant difference was reported between the radioiodine and thyroidectomy groups.

• The nine cats in the methimazole group were not commented on as there were separate to the PICO question of this summary.

• The reference range of T4 was not reported for laboratories used across different private veterinary practices.
• Relatively small sample sizes in each group.
• Radioiodine dose was calculated based on clinical signs rather than uptake studies.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).

• 7/11 (63.6%) cats were clinically and serologically (T4 levels) euthyroid after radioiodine therapy. Six cats were euthyroid after one dose of radioiodine and one cat required a second dose after 6 months.
• 2/11 (18.1%) cats had a partial response to radioiodine therapy where T4 levels dropped but cats remained clinically hyperthyroid.
• 2/11 (18.1%) cats were hypothyroid 1 month and 11 months, respectively, after radioiodine therapy.

• Referral population.
• Four cats did not receive a radioiodine kinetic tracer study to calculate their radioiodine dose so they could have been under- or over-dosed.
• Retrospective data is a weaker form of evidence.
• Small sample size that may be unrepresentative of the general hyperthyroid cat population.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).

• Serum thyroxine (T4) levels.
• Re-evaluation of serum T4 levels 1 months after radioiodine therapy.

• 16/31 (51.6%) cats had T4 within the normal reference interval by post-treatment day 4.
• 21/31 (67.7%) cats had T4 within the normal reference interval by post-treatment day 8.
• 9/31 (29%) cats had decreased T4 levels but were still above the normal reference interval.
• One cat had T4 levels below the normal reference range but did not show clinical signs of hypothyroidism.

• Fairly small population size.
• Referral population.
• Follow-up was only performed for 1 month.

• Serum thyroxine (T4) levels.

• 50/60 (83%) cats had T4 levels within the normal reference interval at re-evaluation.
• 5/60 (8%) were hypothyroid at re-evaluation, without clinical signs. Three of these cats became euthyroid by 6 months post-treatment.
• 5/60 (8%) remained hyperthyroid after treatment, although T4 had significantly reduced post-therapy.

• Referral population.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• Follow-up was only performed for 1 month.

• Total and free T4 concentration.

• 28/32 (87.5%) cats were euthyroid following radioiodine.
• 3/32 (9%) cats remained hyperthyroid after radioiodine
• One cat became hypothyroid after radioiodine treatment. This cat had received bilateral thyroidectomy and a recurrence of hyperthyroid clinical signs 18 months prior to radioiodine.
• The 25 cats surviving remained euthyroid for 1–3 years from the initial radioiodine treatment.

• Small population size.
• Radioiodine dose was calculated based on clinical signs rather than uptake studies.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• A referral population may not represent the general feline population since many cats are only referred due to a difficulty controlling their hyperthyroidism, suggesting more severe disease. Particularly evident in this study since one cat was receiving radioiodine due to a relapse of hyperthyroidism after bilateral thyroidectomy.

• Total plasma thyroxine (T4) concentration pre-radioiodine and 4–23 days after treatment (mainly 10 days after dosing).
• Resolution of clinical signs.

• Euthyroid or hypothyroid cats were not separated.
• No exclusion of non-thyroidal illness or concurrent disease was made.
• Pre-radioiodine antithyroid medication was not considered.
• Referral population.

• Cats were treated with radioiodine (80–00 MBq).
• Radioiodine was injected intravenously in 27 cats and injected subcutaneously in 23 cats.

• Serum thyroxine (T4) concentration.

• 47/50 (94%) cats became euthyroid following radioiodine treatment.
• Significant decrease in serum T4 levels from a mean of 181.3 ± 111.4 nmol/L to a mean of 19.0 ± 29.6 nmol/L 30 days after injection of radioiodine.
• Five cats remained hyperthyroid following treatment but serum T4 levels were decreased compared to pretreatment. Of these cats:
  • Two cats became euthyroid within 3–5 months of therapy.
  • Two cats were lost to follow-up.
  • The remaining cat was treated again after 4 months and become euthyroid.
• No cats became clinically hypothyroid and serum T4 levels below reference range was only transient.
• Recurrence of hyperthyroidism did not happen in any cat in follow-up periods up to 32 months.
• No difference between intravenous or subcutaneous radioiodine injection.

• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• Referral population.
• Variable durations between radioiodine and follow-up samples.

• Total plasma thyroxine (T4) concentration.
• Resolution of clinical signs.

• 200/236 (85%) cats were euthyroid after treatment.
• 10/236 (4%) cats were hyperthyroid after treatment.
• 22/236 (9%) cats were hypothyroid after treatment.

• Retrospective studies are relatively low down on the evidence hierarchy.
• Multivariate analyses may have been more suitable than bivariate analyses for the data in this study.
• Variable durations between radioiodine and follow-up samples.
• Long-term follow-up was often carried out by telephone communications with owners.
• Response to radioiodine was characterised vaguely. Cats were asymptomatic (normal or low T4 with no clinical signs), hyperthyroid (clinical signs and high T4) or hypothyroid (clinical signs and low T4). This does not account for subclinical hypothyroidism and as such the true number of euthyroid cats cannot be determined.
• Referral population.

• 51/60 (85%) cats treated with IV radioiodine were euthyroid 4 years after treatment.
• 50/60 (83%) cats treated with SC radioiodine were euthyroid 4 years after treatment.

• Referral population.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).

• Cats were treated with radioiodine (2–6 mCi equivalent to 74–222 MBq).
• 14 cats had partial thyroidectomy prior to radioiodine treatment.

• Total thyroxine (T4) concentration.

Samples measured pretreatment and again on day of discharge, 2–3 months, 6–12 months and then yearly after radioiodine treatment.

• At discharge, 448/524 (85.5%) cats had serum T4 concentrations within the reference range.
• 2–3 months after treatment, serum T4 concentration was within the reference range in 422/519 (81.3%) cats.
• 6–12 months after treatment, serum T4 concentration was within the reference range in 437/502 (87.1%) cats.
• Overall, the response to treatment was ‘good’ (resolution of clinical signs and T4 levels within the reference range) in 485/515 (94.2%) longer-term.
• 11/524 (2.1%) cats developed clinical hypothyroidism.
• By 6 months post-radioiodine, only 8/524 (1.5%) cats were persistently hyperthyroid.

• T4 levels below the lower reference limit determined hypothyroidism without accounting for T4-lowering effect of non-thyroidal diseases.
• The statistical analysis did not account for the grouping of cats into different groups depending on the severity of hyperthyroidism.
• Referral population.

• Serum total T4 concentration;
• Biochemistry;
• Urinalysis.

• T4 was normal in two cats after 30 days.
• T4 was below normal in 15 cats after 30 days.
• T4 was above normal in five cats after 30 days.
• Response to pretreatment antithyroid medication was not recorded that may affect the population characteristics.
• T4 was normal in 10 cats and above normal in 10 cats that were also evaluated immediately following radioiodine treatment.
• Extended follow-up data was available for 14 cats. 10/14 cats with below normal T4 at 30 days had normal T4 values long-term. Two cats persisted with low T4 values and two cats persisted with high T4 values. No follow-up data was available for the two cats with normal T4 values on day 30.

• Small population size.
• Long-term follow-up was not performed in all cats.
• Referral population.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).

• Thyroxine (T4) concentration.
• Resolution of clinical signs.

• Quantitative values were not reported but 12 month follow-up measurements of T4 were reported in a histogram.
• The number of cats with T4 within the normal reference intervalincreased from approximately 100–150 from 1 week to 6 months after radioiodine treatment. This reduced to approximately 125 cats after 12 months.
• Cats that had elevated T4 levels 1 week after radioiodine treatmentfell to 0 cats at 6 months and 12 months.
• Cats with T4 levels below the reference interval at 1 week after radioiodine treatment were slightly higher by 12 months post- treatment. The specific values were not reported.

• Not all cats were followed-up at every time point. For inclusion, two time points were required out of 1 week and 1, 3, 6 and 12 months post-therapy.
• Retrospective studies are relatively low down on the evidence hierarchy.
• Referral population.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).

• Total thyroxine (T4) concentration;
• Urea and creatinine.

Samples measured pretreatment, 10 days after and several months after treatment.

• After 10 days and several months, T4 had fallen below the upper reference limit in 64/83 (77%) and 72/83 (87%) cats, respectively.
• 4/83 (5%) cats T4 levels decreased below the lower reference limit. Two of these cats had clinical signs of hypothyroidism.

• The analyses did not account for the effect of pre- radioiodine antithyroid drugs. 65/83 (78%) cats had been pretreated with antithyroid medication with a withdrawal of only 3 days prior to radioiodine treatment.
• Samples across the study population were analysed in different laboratories with different reference ranges.
• Referral population.

• Serum total T4 concentration,
• Biochemistry,
• Glomerular filtration rate (GFR).

• Referral population.
• Small population size.
• Cats being medically treated prior to radioiodine were not excluded and a wash-out period for medication was not noted.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).

• Total thyroxine (T4) concentration;
• Biochemistry: Aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), total bilirubin, cholesterol, albumin, blood urea nitrogen (BUN), creatinine, phosphorous, calcium;
• Total protein.

• Only one pretreatment and post-treatment sample was taken. Longer-term sampling would be after follow-up to the response to radioiodine treatment.
• Retrospective studies are relatively low down on the evidence hierarchy.
• Different laboratories were used to measure T4 across the study group, and as such values were reported as a function of the specified normal range of each laboratory.
• Referral population.
• Cats that have adverse reactions to methimazole prior to therapy may response differently to radioiodine to cats that do not have adverse reactions.

• Serum total thyroxine (T4) concentration;
• Blood pressure;
• Haematocrit (HCT), blood urea nitrogen (BUN), creatinine, urine specific gravity (USG), alanine transaminase (ALT), alkaline phosphatase (ALP), albumin, globulin, TLI, folate, cobalamin, methylmalonic acid (MMA).

• Clinical work-up did not completely rule out all concurrent illness.
• Small population size.
• Referral population.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• Resolution of clinical signs was also not measured in follow-up samples. Non-thyroidal illness may have played a role in lowering T4 concentrations.

n = 128 – prospective cohort study.

• Serum total thyroxine (T4) concentration.
• Thyroid volume based of scintigraphic findings.

• 111/167 (66.5%) cats had a total T4 concentration within the normal reference interval at 6 months after therapy.
• 40/167 (24%) cats had a total T4 concentration below the normal reference interval at 6 months after therapy.
• 16/167 (9.5%) cats had a total T4 concentration above the normal reference interval at 6 months after therapy.
• 62/111 (55.9%) of the euthyroid cats were bilaterally affected, 40/111 (36%) were unilaterally affected and 9/111 (8.1%) showed the presence of ectopic thyroid tissue.

• Referral population.
• Retrospective studies are relatively low down on the evidence hierarchy, although the prospective and retrospective groups were controlled well.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• Resolution of clinical signs was also not measured in follow- up samples.
• Non-thyroidal illness may have played a role in artificially lowering T4 concentrations.

• 39 cats received standard dose iodine-131 (I-131).
• 150 cats received low-dose I-131.

• Objective assessment of serum thyroxine (T4) concentration, thyroid stimulating hormone (TSH) and creatinine pre-intervention and 1, 3 and 6 months following radioiodine treatment. Cats were treated with two different I-131 doses (2 mCi or 4 mCi) depending on the severity of hyperthyroidism before treatment.
• Euthyroidism was determined as T4 0–9 – 9 μg/dL or TSH <0.3 ng/mL.

• Cats treated a standard radioiodine dose (4 mCi) had significantly lower serum T4 concentrations and higher TSH at 1, 3 and 6 months, compared with cats treated with lower dose radioiodine (2 mCi).
• Of the cats receiving standard dose (4 mCi) treatment (17/39 were euthyroid after 6 months (44%).
• Of the cats receiving low dose (2 mCi) treatment 113/150 were euthyroid after 6 months (75%).
• Cats treated with the standard dose (4 mCi) were significantly less likely to be euthyroid than those treated with the low dose (2 mCi).
• Cats treated with the lower dose (2 mCi) had lower serum creatinine concentrations and were less likely to develop iatrogenic hypothyroidism.

• The number of animals in each dose group was different that may hide the real effect of dose on radioiodine result.
• Allocation of each dose was not randomised and cats were differed only by the hospital and hospital protocol they were referred to, introducing potential bias.
• Different laboratories were used for pre-intervention and follow-up samples across the study population.
• Referral population.

• Body condition.
• Leptin and ghrelin concentration.
• Serum total thyroxine (T4) concentration.

• 10/42 (24%) cats were euthyroid 4 weeks after radioiodine treatment.
• 28/42 (67%) cats were hypothyroid 4 weeks after radioiodine treatment.
• 4/42 (9%) cats were hyperthyroid 4 weeks after radioiodine treatment.

• Fairly small population size.
• Referral population.

• Physical examination.
• CBC and serum biochemistry.
• Serum total thyroxine (T4) concentration.
• Urinalysis: urine specific gravity (USG), urinary pH, urine protein creatinine ratio, sediment examination and bacterial culture).

• 27/42 (64%) cats had a T4 concentration within the normal reference interval 1 month after radioiodine treatment.
• 15/42 (36%) cats had a T4 concentration below the normal reference interval 1 month after radioiodine treatment.

• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• The wash out period for antithyroid drugs prior to radioiodine treatment was only 10 days.
• Small population size.
• Response to radioiodine was only measured 1 month after. A longer-term follow-up may identify how radioiodine normalises T4 concentration, or not.
• Referral population.

• Serum total thyroxine (TT4) concentration.

• Median serum TT4 concentration significantly reduced at first and second follow-ups compared to diagnosis.
• Two cats developed iatrogenic hypothyroidism.
• Two cats had persistent hyperthyroidism.
• Two cats relapsed into hyperthyroidism 1.6 and 3.4 years after radioiodine.

• Different laboratories measured T4 concentrations across the study population. Although ratios were calculated to reduce the impact of this, a better control would be to use one standardised laboratory throughout the study.
• Ratio-corrected thyroid values cannot be directly compared with other studies and do not stipulate true euthyroidism.
• Subclinical hypothyroidism not determined (no thyroid stimulating hormone [TSH] measurements).
• Retrospective studies are relatively lower down on the evidence hierarchy.
• Referral population.

• Control group using standard dose (n = 23).
• Treatment group based of scintigraphic findings (n = 34).

• At 6 months after treatment, 31/57 (54%) cats were euthyroid and 11/23 (48%) control cats were euthyroid.
• At 6 months after treatment, 17/57 (30%) cats were hypothyroid and 10/23 (43%) control cats were hypothyroid.
• At 6 months after treatment, 9/57 (16%) cats were hyperthyroid and 2/23 (9%) control cats were hyperthyroid.
• Combining both groups, cats that remained hyperthyroid following treatment had the highest initial serum T4 concentrations. Cats that become hypothyroid had the lowest initial serum T4 concentrations. There was no significance in these differences however.
• Most cats becoming euthyroid after treatment were found to have asymmetrical bilateral or unilateral disease.

• Retrospective studies are relatively low down on the evidence hierarchy.
• Longer-term evaluation of thyroid parameters would provide a better idea of continued euthyroidism or relapse to hyperthyroidism.
• Relatively small sample size.
• Referral population.

• Body condition and muscle condition scores.
• Creatinine, urea nitrogen, SDMA concentration.
• Serum total thyroxine (TT4) concentration.
• Thyroid stimulating hormone (TSH) concentration.

• 209/262 (79.8%) cats became euthyroid (serum T4 and TSH concentration within the reference interval).
• 53/262 (20.2%) cats developed iatrogenic hypothyroidism (5 had overt hypothyroidism and 48 had subclinical hypothyroidism).

• Cats with persistent hyperthyroidism were excluded from the study after 3 months.
• The wash out period for antithyroid drugs prior to radioiodine treatment was a minimum of 7 days.
• Referral population.

• Total thyroxine (T4) concentration.
• Clinical signs and clinical examination.

• 39/75 (52%) cats were euthyroid at 6 months after radioiodine therapy.
• 11/75 (15%) cats were persistently hyperthyroid at 6 months after radioiodine therapy.
• 25/75 (33%) cats had a total T4 concentration below the reference interval at 6 months after radioiodine therapy.
• Outcome of radioiodine therapy is likely influenced by a multitude of factors including thyroid volume, multifocal disease, pre-therapy T4 concentration and pre-treatment antithyroid medication.

• Retrospective studies are relatively low down on the evidence hierarchy.
• Referral population.

• Serum thyroxine (T4) concentration.
• Thyroid stimulating hormone (TSH) concentration.
• Blood pressure.
• Full haematology and biochemistry.
• Urinalysis.

• 44/55 (80%) cats were euthyroid prior to radioiodine treatment owing to thyroid stabilisation with anti-thyroid medication and iodine-restricted diets.
• Cats were likely more sensitive to the effects of radio-iodine after ceasing anti-thyroid medication.
• Serum T4 concentration within reference interval in 14/55 (25.5%) cats 19 days after radioiodine treatment.
• Serum T4 concentration was decreased in 33/55 (60%) cats and increased in 6/55 (10.9%) cats 19 days following radioiodine treatment.
• 4/55 (7.3%) cats remained persistently hyperthyroid 6–9 months post-radioiodine.
• 22/55 (40%) cats were euthyroid 6–9 months post- radioiodine.
• 22/55 (40%) cats were overtly hypothyroid 6–9 months post-radioiodine.
• 7/55 (12.7%) cats were subclinically hypothyroid 6–9 months post-radioiodine.

• Small population size.
• Referral population.

• 25 cats completed 1 and 6 month follow-up visit.
• 20 cats completed 12 month follow-up visit.

• Total thyroxine T4 (TT4) concentration and thyroid stimulating hormone (TSH) concentration.
  • Hypothyroidism assessed as TT4 < 15 nmol/L, TSH > 0.15 ng/mL.
  • Subclinical hypothyroidism assessed as TT4 15–60 nmol/L, TSH > 0.15 ng/mL.
  • Euthyroidism assessed as TT4 < 60 nmol/L, TSH < 0.15 ng/mL.
  • Hyperthyroidism assessed as TT4 > 60 nmol/L, TSH < 0.03 ng/mL.
  • The reference boundaries for each group were based on a study by Wakeling et al. (2011).
• Glomerular filtration rate: serum creatinine and slope-intercept iohexol clearance.

• 14/20 (70%) cats were euthyroid 12-months after radioiodine treatment.
• Only 2/20 (10%) were persistently hyperthyroid over 12 months and required a second radioiodine treatment.
• 5/20 (25%) cats were hyperthyroid within 1 month of treatment but were euthyroid by 6 months.
• 4/20 (20%) cats developed overt or subclinical hypothyroidism within 12 months after radioiodine treatment.

• Small study power reduces the power of the evidence.
• The study only included cats with mild-moderate hyperthyroidism and as such, they only received a low-dose of radioiodine therapy (111 MBq).
• Referral population.

• Serum thyroxine (T4) concentration.
• Haematology and biochemistry.
• Coagulation panel: fibrinogen, partial thromboplastin time (PT) and activated partial thromboplastin time (aPTT), thromboelastography.

• Median T4 concentration significantly decreased in hyperthyroid cats 7 days and 14 days after radioiodine treatment.
• After 14 days, 7/15 cats (46.7%) had normal T4 and 8/15 (53.3%) had low T4.

• T4 was only followed up over 14 days following radioiodine administration.
• Small population size reduces the power of the evidence.
• Since TSH was not evaluated 1–3 months after treatment as per the guidelines, euthyroid cats could not be distinguished from subclinical or overtly hypothyroid cats. The terms ‘low T4’ and ‘normal T4’ were used.
• Although concurrent disease was an exclusion criterion for the study population, radiation-induced thyroiditis could influence thyroid hormone levels in the short-term. An extended study would be needed to determine absolute T4 levels in response to radioiodine treatment.
• Referral population.

• Total thyroxine T4 concentration.
• Serum creatinine concentration and urinalysis, where available.

Samples measured pre-radioiodine and at least 1 month after.

• At re-check, 133/161 (82.6%) cats had a total T4 concentration within the reference interval.
• At re-check, 4/161 (2.5%) cats had a total T4 concentration above the reference interval – persistent hyperthyroidism.
• At re-check, 24/161 (14.9%) cats had a total T4 concentration below the reference interval.

• Samples across the study population were analysed in different laboratories with different reference ranges.
• Only T4 concentration was used as a marker the severity of hyperthyroidism without considering other factors including goitre size or clinical signs.
• The duration between diagnosis to radioiodine treatment and duration between radioiodine and follow-up varied substanially between cats. It may be possible that relapse into hyperthyroidism or longer-term thyroid changes were missed or inconsistent.
• Retrospective studies are relatively low down on the evidence hierarchy.
• Referral population.

• Hyperthyroid cats = 49 Healthy control cats = 45
• Chronic kidney disease = 9
• Cats with concurrent CKD and hyperthyroidism at time of diagnosis were excluded (n = 6).

• Total thyroxine T4 concentration and thyroid stimulating hormone (TSH) levels were measured at diagnosis (T0), then 1 month after radioiodine treatment (T1) and again 11–29 months after radioiodine treatment (T2).
• Hyperthyroidism was diagnosed based on compatible clinical signs, increased serum T4 levels or thyroid uptake of pertechnetate.
• Serum creatinine concentration and urinalysis, where available.
• Thoracic radiographs and abdominal ultrasound to identify concurrent disease.
• Complete blood count (CBC).

• 49/49 cats had T4 levels recorded at T0 and T1. 26/49 cats had T4 levels recorded at T2 (23/49 cats were lost to follow-up due to owner non-compliance or death before assessment).
• At T0 all cats were hyperthyroid.
• At T1, 41/49 (84%) cats were euthyroid, 4/49 (8%) cats were sub clinically hypothyroid, 1/49 (2%) cat was hypothyroid, 1/49 (2%) cat remained hyperthyroid and 2/49 (4%) cats had uncertain thyroid status because they were missing TSH values.
• At T2, there were 18/26 (69%) euthyroid cats, 7/26 (27%) sub clinically hypothyroid cats and 1/26 (4%) iatrogenic hyperthyroid cat (levothyroxine overdose).
• 2/26 (8%) cats developed post-treatment azotaemia at T2.

• 23/49 cats were lost to follow-up at T2 due to owner non-compliance (n = 21) or death (n = 2).
• T2 varied greatly between 11–29 months.
• Referral population.

Cats with mild disease received < 66.6 MBq.

Cats with moderate disease received 66.6–92.5 MBq. Cats with severe disease received > 92.5 MBq.

• Complete blood count (CBC), biochemistry and urinalysis.
• Total thyroxine T4 concentration.
• T3 concentration.
• Thyroid stimulating hormone (TSH) concentration.
• Qualitative and quantitative thyroid scintigraphy.

• 1047/1400 (74.8%) cats became euthyroid.
• 57/1400 (4.1%) cats became overtly hypothyroid.
• 240/1400 (17.1%) became sub-clinically hypothyroid.
• 56/1400 (4%) cats remained hyperthyroid.
• Cats with sub-clinical and overt hypothyroidism were older than euthyroid cats.
• Cats with persistent hyperthyroidism were younger than euthyroid cats.
• Hyperthyroid cats with bilateral disease were 1.5 times more likely to develop radioiodine-induced hypothyroidism than cats with unilateral nodules.

• Referral population.
• No comparison made with other radioiodine protocols.

• Complete blood count (CBC), biochemistry and urinalysis.
• Total thyroxine T4 concentration.
• Triiodothyronine (T3) concentration.
• TSH concentration.
• Qualitative and quantitative thyroid scintigraphy
• Measurements taken before and 6–12 months after radioiodine therapy.

Cats were treated with a single dose of individualised radioiodine that was calculated taking into account patient-specific dosing variables.

Cats with mild received < 66.6 MBq.

Cats with moderate disease received 66.6–92.5 MBq.

Cats with severe disease received > 92.5 MBq.

• 1047/1400 (74.8%) cats become euthyroid 6–12 months following radioiodine therapy.
• 57/1400 (4.1%) cats become overtly hypothyroid 6–12 months following radioiodine therapy.
• 240/1400 (17.1%) cats become sub-clinically hypothyroid 6–12 months following radioiodine therapy.
• 56/1400 (4%) cats remained persistently hyperthyroid 6–12 months following radioiodine therapy.
• More overtly 41/57 (71.9%) and sub clinically 95/240 (39.6%) hypothyroid cats became azotaemic compared to euthyroid cats 149/1047 (14.2%).

• Referral population.
• No comparison made between other radioiodine protocols.
• Underestimation of azotaemic cats was possible as muscle wastage may cause a falsely low serum creatinine.