Thyroid Stimulating Hormone Blood Test

The main role of the thyroid is to produce key hormones that affect almost every cell in the body. Thyroid hormones thyroxine (T4), and triiodothyronine (T3) are vital in regulating the rate at which the body uses energy. Thyroid stimulating hormone (TSH) is produced in the pituitary gland and regulates the production of T3 and T4. It is often measured as a proxy for thyroid hormones T4 and T3 as when circulating hormones are low, TSH will be elevated, and when circulating hormones are high TSH will be low.

Raised levels of TSH can indicate that the thyroid is struggling to produce sufficient quantities of thyroid hormones and is underactive, whereas low levels can indicate that the thyroid is producing excess levels of thyroid hormones and is overactive. Symptoms of an overactive thyroid (hyperthyroidism) include heat intolerance, weight loss, heart palpitations and breathlessness, while common underactive thyroid (hypothyroidism) symptoms include fatigue, weakness, weight gain, slow heart rate and cold intolerance.

The TSH test is often the first test for evaluating thyroid function and/or symptoms of hyper or hypothyroidism.