Thyroid Hormone Physiology and Regulation

Thyroid hormones influence cellular energy use throughout the body. Their effects appear in heart rate, heat production, bowel motility, cognition, growth, reproductive function, and bone turnover. A small change in circulating hormone can therefore create findings across several body systems. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to synthesize and release thyroxine (T4) and triiodothyronine (T3). This axis works by negative feedback. As circulating free T4 and T3 rise, they suppress pituitary TSH release. When thyroid hormone falls, TSH rises to push the gland to produce more hormone. TSH functions like the thermostat’s signal to the furnace. In primary hypothyroidism, the thyroid gland cannot produce enough hormone, so the pituitary raises TSH in an attempt to stimulate it. In primary hyperthyroidism, excess thyroid hormone suppresses TSH. This logic changes when the pituitary or hypothalamus is diseased. A low or inappropriately normal TSH does not exclude hypothyroidism if free T4 is low, because the gland may not be receiving an adequate TSH signal. TSH results must...