Research overview: laboratory diagnosis of feline hyperthyroidism

Diagnosis of feline hyperthyroidism rests primarily on measuring elevated total thyroxine (T4), supported in equivocal cases by free T4, repeat testing, or dynamic tests such as T3 suppression and thyroid scintigraphy. Diagnostic nuance arises because concurrent nonthyroidal illness can suppress T4 into the normal range, masking early or mild disease in sick older cats. This overview compiles the diagnostic literature rather than one source, describing the hierarchy of tests, why a normal T4 does not always exclude the disease in clinically suspicious cats, and how scintigraphy can localize and characterize functional thyroid tissue. It matters because accurate diagnosis distinguishes hyperthyroidism from other causes of weight loss in older cats and guides selection among the available curative and management options.