The thyroid gland secretes two main hormones into the bloodstream. One of these is thyroxine, which contains four atoms of iodine and is often called T4. This, in turn, is converted to tri-iodothyronine (T3), which contains three atoms of iodine. It is the T3 that is biologically active and regulates metabolism. This article provides a general guide for the thyroid function test in laboratories.
The amount of T4 and T3 secreted by the thyroid gland is regulated by the pituitary gland, which lies underneath your brain. The pituitary senses the level of thyroid hormones in bloodstream by a similar mechanism that of a thermostat. If the level drops just a little below normal, the pituitary reacts by secreting a hormone called thyroid-stimulating hormone (TSH), which activates the thyroid gland to produce more T4. When the thyroid hormone levels rise above normal, the ‘thermostat’ senses…
