see also:

• endocrinology
• hyperthyroidism
• hypothyroidism
• American Thyroid Association guidelines

• the hypothalamus releases thyrotropin releasing hormone (TRH)
• TRH stimulates the anterior pituitary to release thyroid stimulating hormone (TSH)
• 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

• 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⁾