T4 test

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Blood test

1. Thyroid-stimulating hormone (TSH) is a secreted by the pituitary gland.
2. TSH causes the thyroid gland to produce two more hormones, T4 (thyroxine) and T3 (triiodothyronine).
3. Finally, TSH itself is stimulated by another hormone, thyroid-releasing hormone (TRH), which is made by the hypothalamus.

In people with normal thyroid function, having enough T3 and T4 inhibits both TSH and TRH, which prevents the body from making too much T3 and T4.

T4 levels are important, because T4 increases numerous enzymes that produce energy for the body.

Most T3 and T4 is transported by a protein called TBG (thyroxine binding globulin), but smaller amounts are found on prealbumin and albumin. When not bound to proteins, they are called "free" T3 or T4.

Review Date: 01/18/2006
Reviewed By: Nikheel S. Kolatkar, MD, Clinical and Research Fellow, Division of Endocrinology, Diabetes, and Hypertension, Brigham and Women's Hospital, Harvard Medical School, Boston, MA. Review provided by VeriMed Healthcare Network.