Update on thyroid function tests in cats: Where does TSH fit in?
Great strides have been made in the recognition of feline hyperthyroidism since its first description as a clinical entity over 25 years ago. However, there remain significant issues with regard to the sensitivity and specificity of the available thyroid function tests for the cat. Also, given that hyperthyroidism is a relatively new disease of cats and has been associated with environmental and nutritional risk factors, it is of great interest to develop a battery of thyroid function tests which would improve our understanding of the feline hypothalamic-pituitary-thyroid axis to these endocrine disruptors. The key test still lacking wide availability is a feline-specific immunoassay for thyrotropin (TSH). Hypotheses for the pathophysiological causes of hyperthyroidism would be considerably narrowed by demonstration of consistent and early alteration of serum TSH concentrations. Elevation in response to a test compound would support a goitrogenic effect and indicate that TSH elevation had an intermediary role in the genesis of thyroid adenomatous hyperplasia. Conversely, the early diagnosis of hyperthyroidism would be facilitated by demonstration of suppression of TSH. However, although the diagnostic sensitivity of the human TSH assay allows distinction of normal from low values, the canine TSH assay does not have this level of sensitivity. An optimal feline TSH assay system would hopefully distinguish normal from low values. Such sensitivity will be critical as practitioners are already recognizing cats with palpable thyroid glands but normal total and free T4 concentrations. Also, the diagnostic difficulties associated with sick hyperthyroid cats would presumably be alleviated.