Correlation of thyroid hormone measurements with thyroid stimulating hormone stimulation test results in radioiodine-treated cats

Utility of a canine TSH assay for diagnosis and monitoring of feline hyperthyroidism

OBJECTIVE

This retrospective study was initiated to evaluate the utility of TSH measurements using a common canine TSH assay to diagnose and monitor feline hyperthyroidism after radioiodine or thyreostatic drug treatment.

MATERIAL AND METHODS

The electronic database of the University of Veterinary Medicine Vienna was searched for combined TSH and T4 measurements. 217 pairs of TSH and T4 from 136 cats with possible hyperthyroidism were assigned to group A (untreated; n = 24) and B (treated; n = 193). Measurements in group B were then subcategorized according to T4 concentrations (reference range 15-50 nmol/L): group B1 = elevated T4 (n = 46), group B2 = normal T4 (n = 84) and group B3 = decreased T4 (n = 63). Group B2 was further divided into cats with low normal (group B2a; n = 35), medium normal (group B2b; n = 29) and high normal (group B2c; n = 20) T4 concentrations.

RESULTS

TSH was detectable in 4 (17 %) of the 24 untreated cats (group A) and did not return to normal despite seemingly successful therapy in two. Increased TSH concentrations were observed in 3.6 % of the treated cats in group B2 and 2.9 %, 6.9 %, and 0 % in subgroups B2a, B2b and B2c, respectively. Forty-four percent of the treated cats with a decreased T4 (group B3) had an increased TSH concentration. TSH correlated with treatment length (r = 0.358, p = 0.004) and was significantly higher in cats treated for more than 3 months (p = 0.008).

CONCLUSION

TSH was detectable in a significant number of untreated hyperthyroid cats and thus this parameter should not be used to definitively rule out feline hyperthyroidism. Furthermore, the very low prevalence of increased TSH concentrations in treated hyperthyroid cats with a normal T4 and cost benefit calculations do not support the routine measurement of TSH in these cats. The fact that TSH correlated with time since treatment start and 56 % of the cats with a decreased T4 had TSH concentrations within the reference limits, suggests delayed recovery of the pituitary thyrotrophs which might explain the low prevalence of subclinical hypothyroidism in the present study.

CLINICAL RELEVANCE

TSH measurement in cats with suspected or treated hyperthyroidism using a canine assay lacks diagnostic sensitivity and can only complement therapeutic decision-making.

Outcome of radioiodine therapy for feline hyperthyroidism: Fixed dose versus individualized dose based on a clinical scoring system

Background

Hyperthyroidism is the most frequent endocrinopathy in older cats. To date, there is no consensus on how to best calculate the dose of radioiodine to administer to hyperthyroid cats.

Aim

The goals of this study were to compare thyroid function, renal function, and survival time between hyperthyroid cats receiving a fixed dose of radioiodine and those receiving an individualized dose calculated using a clinical scoring system.

Methods

Medical records of 110 cats treated with radioiodine therapy at the University of Bern between 2010 and 2020 were reviewed. Thyroid function, renal function, and survival of cats treated with a fixed dose of radioiodine (2010-2015; n = 50) were compared to those of cats treated with an individualized dose (2015-2020; n = 60) at different time points after therapy.

Results

Treatment with a fixed dose of radioiodine (mean = 168 ± 26 MBq) was associated with 69% of euthyroidism, 19% persistent hyperthyroidism, and 12% hypothyroidism, whereas treatment with an individualized dose (mean = 120 ± 30 MBq) led to 54% euthyroidism, 23% hyperthyroidism, and 23% hypothyroidism (p = 0.73). More than 12 months after treatment, the incidence of azotemia was comparable between cats treated with a fixed dose (37%) and those treated with an individualized dose (31%) (p = 0.77). No factors were found to be predictive of treatment failure (hypothyroidism or hyperthyroidism) after therapy. Median survival time after radioiodine therapy was 44 months. In a multivariate analysis, persistent hyperthyroidism was the only variable independently associated with a shorter survival time (HR = 6.24, p = 0.002).

Conclusion

The method of calculating the dose of radioiodine (fixed vs. individualized) to treat feline hyperthyroidism does not appear to be decisive for posttreatment thyroid function, renal function, or survival.

Correlation of thyroid hormone measurements with thyroid stimulating hormone stimulation test results in radioiodine-treated cats

Background

Iatrogenic hypothyroidism can develop after radioiodine‐I¹³¹ (RAI) treatment of hyperthyroid cats and can be diagnosed using the thyroid stimulating hormone (TSH) stimulation test.

Objectives

To assess the effect of noncritical illness on TSH stimulation test results in euthyroid and RAI‐treated cats. To assess the correlation of low total‐thyroxine (tT4), low free‐thyroxine (fT4), and high TSH concentrations with TSH stimulation test results.

Animals

Thirty‐three euthyroid adult cats and 118 client‐owned cats previously treated with RAI.

Methods

Total‐thyroxine, fT4, and TSH were measured, and a TSH stimulation test was performed in all cats. Euthyroid control cats were divided into apparently healthy and noncritical illness groups. RAI‐treated cats were divided into RAI‐hypothyroid (after‐stimulation tT4 ≤ 1.5 μg/dL), RAI‐euthyroid (after‐stimulation tT4 ≥ 2.3 μg/dL OR after‐stimulation tT4 1.5‐2.3 μg/dL and before : after tT4 ratio > 1.5), and RAI‐equivocal (after stimulation tT4 1.5‐2.3 μg/dL and tT4 ratio < 1.5) groups.

Results

Noncritical illness did not significantly affect the tT4 following TSH stimulation in euthyroid (P = .38) or RAI‐treated cats (P = .54). There were 21 cats in the RAI‐equivocal group. Twenty‐two (85%) RAI‐hypothyroid cats (n = 26) and 10/71 (14%) of RAI‐euthyroid cats had high TSH (≥0.3 ng/mL). Twenty‐three (88%) RAI‐hypothyroid cats had low fT4 (<0.70 ng/dL). Of the 5 (7%) RAI‐euthyroid cats with low fT4, only one also had high TSH. Only 5/26 (19%) RAI‐hypothyroid cats had tT4 below the laboratory reference interval (<0.78 μg/dL).

Conclusions and Clinical Relevance

The veterinary‐specific chemiluminescent fT4 immunoassay and canine‐specific TSH immunoassay can be used to aid in the diagnosis of iatrogenic hypothyroidism in cats.