Understanding thyroid regulation is important, as thyroid dysfunction or disease is common and complex.
Regulation of Thyroid Function: Thyroid function and metabolism is controlled by the human hypothalamic-pituitary-thyroid (HPT) axis, which is made up of the hypothalamus, the pituitary gland, and the thyroid. Thyroid stimulating hormone (TSH) is released from the pituitary gland, which is attached to the brain. TSH controls the production of thyroid hormones thyroxine (T4) and triiodothyronine (T3), which play an important role in the regulation of metabolism. When the hypothalamus in the brain senses low levels of T4/T3 hormones in the blood it releases thyrotropin-releasing hormone (TRH), which controls TSH release from the pituitary gland. This causes T4/T3 levels to rise. Once the hypothalamus senses that T4/T3 levels are stable, it stops TRH release, which then stops TSH secretion.
Total T4/T3 vs. Free T4/T3: Most of the T4/T3 circulates in the blood attached to protein. The small percentage that is not attached to proteins is considered “free.” Free T4/T3 are active because they aren’t attached to protein and can enter their target tissues. Therefore, it is thought that free T4/T3 may be a more accurate way to measure thyroid hormone function.
Disease/Symptoms: TSH, T4 and T3 are intricately connected and are important measurements to be made when determining or preventing thyroid-related disease or dysfunction. Failure at any level of regulation of the HTP axis can result in thyroid-related dysfunction.