By A.P. Mentzer | Updated Aug 30, 2022
TSH is released by the pituitary gland to stimulate the thyroid to produce thyroxine (T4) whenever circulating T4 levels fall.
The thyroid, a butterfly‑shaped organ positioned in the neck above the trachea, secretes hormones that control core metabolic functions—including heart rate, basal body temperature, respiration, and digestion. The amount of hormone released directly influences how quickly these systems operate.
Its principal product is thyroxine (T4), a molecule that contains four iodine atoms. A smaller amount of triiodothyronine (T3)—which has three iodine atoms—is also produced. In target tissues such as the liver and brain, T4 is converted to the more biologically active T3, although T4 remains the predominant circulating hormone.
Read more about T3 and T4.
TSH’s role is governed by a tight feedback loop with the pituitary gland, located just beneath the hypothalamus. The pituitary monitors blood T4 levels and secretes TSH when T4 is low, prompting the thyroid to ramp up hormone production.
When T4 concentrations rise, the pituitary reduces TSH output, maintaining equilibrium. This dynamic ensures that metabolic processes remain within a healthy range.
Read more about what causes thyroid failure.
Clinicians often order a TSH blood test to evaluate thyroid function, especially when patients exhibit symptoms suggestive of abnormal metabolism. Deviations from the normal TSH range can indicate either hypothyroidism or hyperthyroidism.
A low T4 coupled with a high TSH points to hypothyroidism, an underactive thyroid that slows bodily functions. Common signs include fatigue, constipation, weight gain, cold intolerance, bradycardia, and depression.
Conversely, a high T4 paired with a low TSH suggests hyperthyroidism, an overactive thyroid that accelerates metabolism. Symptoms often involve insomnia, frequent bowel movements, weight loss, overheating, tachycardia, and jitteriness.
