Laboratory Diagnosis of Thyroid and Adrenal Disease

Diagnosis of thyroid and adrenal disease can be confusing. Whereas the definitive diagnosis of hyperthyroidism and hypoadrenocorticism are relatively straightforward, hypothyroidism and naturally occurring Cushing's syndrome (NOCS) are more complicated. In a patient with compatible clinical signs, a single increased tT4 is enough to confirm hyperthyroidism, but a low tT4 alone is never enough to confirm hypothyroidism. A flatline result (post-stimulation cortisol <2 ug/dL) on an ACTH stimulation test (ACTHst) confirms hypoadrenocorticism, but not all dogs with NOCS have increased ACTHst results. This article explains which diagnostics should be pursued for these endocrinopathies, and how to interpret them.