Evaluation of Serum Thyroid‐Stimulating Hormone Concentration as a Diagnostic Test for Hyperthyroidism in Cats

Abstract

BackgroundIn humans, measurement of serum thyroid‐stimulating hormone (TSH) concentration is commonly used as a first‐line discriminatory test of thyroid function. Recent reports indicate that canine TSH (cTSH) assays can be used to measure feline TSH and results can help diagnose or exclude hyperthyroidism.ObjectivesTo investigate the usefulness of cTSH measurements as a diagnostic test for cats with hyperthyroidism.AnimalsNine hundred and seventeen cats with untreated hyperthyroidism, 32 euthyroid cats suspected of having hyperthyroidism, and 131 clinically normal cats.MethodsProspective study. Cats referred to the Animal Endocrine Clinic for suspected hyperthyroidism were evaluated with serum T4, T3, free T4 (fT 4), and TSH concentrations. Thyroid scintigraphy was used as the gold standard to confirm or exclude hyperthyroidism.ResultsMedian serum TSH concentration in the hyperthyroid cats (<0.03 ng/mL) was significantly (P < .001) lower than concentrations in clinically normal cats (0.05 ng/mL) or euthyroid cats with suspected thyroid disease (0.06 ng/mL). Only 18 (2.0%) hyperthyroid cats had measurable TSH concentrations (≥0.03 ng/mL), whereas 114 (69.9%) of the 163 euthyroid cats had detectable concentrations. Combining serum TSH with T4 or fT 4 concentrations lowered the test sensitivity of TSH from 98.0 to 97.0%, but markedly increased overall test specificity (from 69.9 to 98.8%).Conclusions and Clinical ImportanceSerum TSH concentrations are suppressed in 98% of hyperthyroid cats, but concentrations are measurable in a few cats with mild‐to‐moderate hyperthyroidism. Measurement of serum TSH represents a highly sensitive but poorly specific test for diagnosis of hyperthyroidism and is best measured in combination with T4 and fT 4.

Evaluation of Quantitative Thyroid Scintigraphy for Diagnosis and Staging of Disease Severity in Cats With Hyperthyroidism: Comparison of the Percent Thyroidal Uptake of Pertechnetate to Thyroid‐to‐salivary Ratio and Thyroid‐to‐background Ratios

“…In these cats, the tentative diagnosis of hyperthyroidism had been established by the primary referring veterinarian on the basis of clinical signs consistent with the disease (e.g., weight loss despite a good appetite), the finding of a palpable thyroid nodule(s) on physical examination, and the finding of high serum thyroid hormone concentrations (T 4 or fT 4 or both). We confirmed hyperthyroidism in all these cats by documenting high serum concentrations of T 4 or fT 4 (usually both), together with undetectable concentrations of serum TSH . Additionally, all of these cats responded positively to treatment with radioiodine.…”

“…Cats that had been fed a low‐iodine diet (y/d, Hills Pet Nutrition, Topeka, KS) within 3 months of evaluation were excluded. Results of the serum thyroid hormone panel were used to confirm or exclude hyperthyroidism (see below), but the results of thyroid scintigraphy were not used to classify cats as euthyroid or hyperthyroid for the purpose of this study.…”

“…These cats were referred for evaluation of possible hyperthyroidism because of suggestive clinical signs (e.g., weight loss despite good appetite, chronic diarrhea), physical examination findings (e.g., palpable mass in cervical region), or thyroid function tests (e.g., high fT 4 concentration). However, these cats were classified as euthyroid based on the findings of normal serum thyroid hormone concentrations on repeat testing, together with detectable concentrations of serum TSH . None of these cats ever had high serum concentrations of either T 4 or fT 4 concurrent with undetectable concentrations of TSH …”

Hyperthyroidism is not a risk factor for subclinical bacteriuria in cats: A prospective cohort study

“…All hyperthyroid cats referred to our clinic for treatment with radioiodine over the 22‐month period from January 2018 to October 2019 were evaluated for inclusion in this prospective cohort study. To be eligible for inclusion, hyperthyroid cats underwent a thorough evaluation that included review of past medical history, complete physical examination (including body weight, body, and muscle condition scoring), 14 routine laboratory testing (CBC, serum biochemical profile, and complete urinalysis), urine culture, determination of serum thyroid hormone concentrations (total T 4 and TSH), 15,16 and qualitative and quantitative thyroid scintigraphy 17 . In cats previously treated with methimazole, the drug was discontinued ≥7 days before evaluation.…”

Efficacy of Low‐dose (2 millicurie) versus Standard‐dose (4 millicurie) Radioiodine Treatment for Cats with Mild‐to‐Moderate Hyperthyroidism

“…Cats were eligible for enrollment if they had mild‐to‐moderate hyperthyroidism (serum T 4 concentration ≥4 μg/dL but <13 μg/dL; reference range 0.9–3.9 μg/dL) together with compatible clinical features (e.g, weight loss despite a good appetite, palpable goiter) 18. Cats with serum T4 ≥ 13 μg/dL were excluded to focus on cats with mild‐to‐moderate hyperthyroidism that might be expected to be cured with a low dose of 131 I 1.…”