TSH stimulates the thyroid to produce thyroxine (T4)
T4 is converted peripherally to T3 which is more active than T4
T3 binds to nuclear receptor proteins that function as transcription factors to regulate the expression of many genes. Nongenomic actions of thyroid hormone also regulate important physiologic parameters.
T3 exerts widespread metabolic effects including:
increases tissue thermogenesis and thus basal metabolic rate (BMR)
reduces serum cholesterol levels
reduces systemic vascular resistance
T4 inhibits further release of TRH and TSH in a negative feedback loop
testing for thyroid disease
patients with hyperthyroidism generally have a low TSH, while those with hypothyroidism generally have a high TSH, thus, for most patients a TSH is all that is needed to screen for thyroid disorder.
high pre-test probability patients with a normal TSH should also have a free T4 level performed, while those with suspected hyperthyroidism with a low TSH but normal free T4, should have T3 testing added.
screening in patients with atrial fibrillation is low yield with clinical hyperthyroidism being present in ~1% while clinical hypothyroidism is present in ~1.5%1)