Thyroid Function Tests

The hypothalamus, pituitary gland, and thyroid form a feedback loop. The anterior pituitary releases thyroid-stimulating hormone (TSH), which signals the thyroid gland to produce thyroxine (T4) and triiodothyronine (T3). Circulating thyroid hormone then feeds back to the pituitary: more free T4 and T3 suppress TSH release, while less hormone allows TSH to rise. TSH behaves like a highly sensitive thermostat. A small fall in circulating free thyroid hormone triggers a much larger rise in TSH, often before free T4 falls outside the reference interval. For this reason, TSH is usually the best first test when the hypothalamic-pituitary axis is functioning normally. In primary thyroid disease, the gland itself is failing or overproducing hormone, so TSH usually moves in the opposite direction from thyroid hormone: - A high TSH usually means the thyroid is not producing enough hormone. - A low or suppressed TSH usually means excess thyroid hormone is present. That pattern fails when the problem is central rather than thyroidal. With pituitary or hypothalamic dysfunction, the pituitary may not produce an appropriate TSH response. A patient can have a...