Can thyroid stimulating hormone (TSH) be used to diagnose hypothyroidism and hyperthyroidism?
Thyroid stimulating hormone (TSH) is released by the pituitary in response to low serum T4 levels, due to loss of negative feedback. An increase in TSH concentration therefore indicates a failure of adequate thyroid hormone production (hypothyroidism). Abnormally high concentrations of T4, as seen in hyperthyroidism (a very rare condition in the dog) will inhibit TSH secretion and cause low serum concentrations of TSH. Unfortunately, current assays for canine TSH do not have sufficient sensitivity to differentiate normal from low concentrations. It is for this reason that the low end of the canine reference range extends to zero and the test cannot be used reliably to identify hyperthyroid states.