More Than Just T₄

Abstract

In this article the author reviews the use of commonly recommended thyroid function tests, focusing on clinical scenarios that present diagnostic difficulties. In doing so, he draws on the veterinary and comparative literature, his own clinical experience, and data, unpublished to date, obtained from a series of 100 hyperthyroid cats consecutively diagnosed at his clinic.

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

“…The total T 4 test, with its relatively high sensitivity and specificity remains the initial screening test of choice for hyperthyroidism in cats 1, 2, 3. Serum T 3 measurements are less useful for diagnosis because of the fact that one‐third of hyperthyroid cats maintain normal serum T 3 concentrations, despite having clearly high total and fT 4 concentrations 1, 2, 3. The fT 4 (equilibrium dialysis) test has very high sensitivity but low specificity (approximately 15% false‐positive cats in this study).…”

“…Cats with suspected hyperthyroidism routinely are screened by measuring T 4 because it is inexpensive and specific. However, up to 10% of all hyperthyroid cats, and over 30% of cats with early or mild hyperthyroidism, have T 4 concentrations that remain within the reference interval 2, 3. Concurrent nonthyroidal illness also can suppress high serum T 4 concentrations to within the reference interval 2, 4, 5, 6…”

“…Serum free T 4 (fT 4 ) concentrations are high in approximately 95% of cats with occult hyperthyroidism and can aid in the diagnosis 2, 3, 6. However, the specificity of fT 4 is relatively poor, with up to 20% of sick (and some clinically normal) euthyroid cats having increased serum fT 4 concentrations 2, 5, 7.…”

“…However, the specificity of fT 4 is relatively poor, with up to 20% of sick (and some clinically normal) euthyroid cats having increased serum fT 4 concentrations 2, 5, 7. Tests of the hypothalamic‐pituitary‐thyroid (HPT) axis (eg, T 3 suppression test) can help in diagnosis of occult hyperthyroidism but suffer from problems with owner compliance and equivocal test results 3, 6, 8. Consequently, quantitative thyroid scintigraphy, with calculation of thyroid‐to‐salivary and thyroid‐to‐background ratios, is considered the most accurate test for occult hyperthyroidism in cats 9.…”

ANIMAL MODELS OF DISEASE: Feline hyperthyroidism: an animal model for toxic nodular goiter

“…2). The histologic appearance of this paranodular tissue resembles that of thyroid glands obtained from animals treated with levothyroxine (L-T 4 ) and reflects the fact that circulating thyroid-stimulating hormone (TSH) is suppressed in these cats as a result of their chronic hyperthyroid state , Capen 2001, Peterson 2013b.…”