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

Abstract

Background: Iatrogenic hypothyroidism can develop after radioiodine-I 131 (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 RAIequivocal (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%) RAIhypothyroid 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).

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

“…As one of these cats was clinically hypothyroid, only 2 of these cats possibly had “subclinical hypothyroidism”. “Possibly” has to be emphasized as, although the specificity of an increased TSH was 98.5 % and thus excellent in the initial study of Peterson et al 11 , 10 of 71 (14 %) and 6 of 22 (27 %) of treated “euthyroid” cats had misleading increased TSH concentrations in later studies 12 , 16 . While a single TSH measurement might be rather inexpensive (18 €), the very low prevalence of elevated TSH concentrations in cats with a normal T4 and cost benefit calculation of all TSH measurements in group B2 (1476 € for the detection of 2 possible cases of subclinical hypothyroidism), do not support the routine integration of TSH measurement in cats with a normal T4 concentration and no clinical evidence of hypothyroidism.…”

“…Unfortunately, iatrogenic hypothyroidism, which is associated with progression of renal disease and increased morbidity and mortality 10 , 11 , 12 often remains undetected due to lack of clinical signs and T4 values within the reference range 12 , 28 . Possible reference tests for detecting iatrogenic feline hypothyroidism are thyroid scintigraphy 11 and the TSH-stimulation test 12 . While thyroid scintigraphy is only available in a few veterinary clinics throughout Europe and requires general anesthesia, TSH stimulation test is time consuming, and recombinant human TSH is very expensive prohibiting routine use in small animal practice 29 , 30 .…”

“…Further recombinant human thyrotropin is not licensed for veterinary use in Austria, Germany or Switzerland. In contrast, TSH determination is a comparably inexpensive measure and can be requested from previously submitted serum samples when needed because of the high stability of TSH [9] [12]. The current recommendation of the American Association of Feline Practitioners (AAFP) is to aim for T4 concentrations in the lower half of the reference interval in cats without renal disease, but to "ensure a normal TSH" [3].…”

“…The use of T4 as a monitoring tool to categorize treatment success implies that this parameter reliably identifies cats with iatrogenic hypothyroidism, which is extremely common in radioiodine (overt [1 %–40 %] and subclinical [12.7 %–47 %]) 7 , 8 or thiamazole (20 %) 9 treated hyperthyroid cats, and is associated with deterioration of renal function and reduced survival time 10 , 11 . Alarmingly, recent studies using scintigraphy 11 or the TSH-stimulation test 12 as a reference test to identify overtreatment showed that total T4 is an insensitive marker to identify radioiodine induced hypothyroidism in cats. T4 was within the reference range in 53.6 % and 81 % of likely hypothyroid cats and thus the authors reinforced the need for additional monitoring tools 12 .…”

“…Alarmingly, recent studies using scintigraphy [11] or the TSH-stimulation test [12] as a reference test to identify overtreatment showed that total T4 is an insensitive marker to identify radioiodine induced hypothyroidism in cats. T4 was within the reference range in 53.6 % and 81 % of likely hypothyroid cats and thus the authors reinforced the need for additional monitoring tools [12].…”