Thyrothytropin (TSH), which plays a key role in the secretion of thyroid hormones, is secreted from the pituitary gland. TSH secretion is stimulated by the secretion of thyrotropin releazing hormone (TRH) from the hypothalamus. TSH secretion usually peaks between midnight and 04:00, depending on our body’s natural diurnal rhythm.
TSH increases the level of thyroid hormone. When the thyroid hormone is not secreted enough, TSH secretion is stimulated. TSH is secreted more and raises its own level in order to increase the level of thyroid hormone. If TSH secretion is not sufficient in pituitary gland diseases, low thyroid hormones also occur. In some diseases such as tumors of the pituitary gland, if TSH secretion increases abnormally, increases in all thyroid hormones continue.
The thyroid secretes T4 and T3 hormones. These hormones are also stimulated by the secretion of TSH. When the TSH, T3 and T4 tests have low levels of all three, the problem is usually caused by the pituitary gland. When TSH is normal or high, low T3 and T4 levels usually indicate thyroid gland diseases.
TSH binds to its receptors on the membranes of thyroid cells and activates adenylate cyclase in the cell, which causes an increase in cyclic adenosine monophosphate (cAMP) levels and initiation of thyroid hormone secretion. Afterwards, iodines coming from the blood or stored in the thyroid are combined and thyroid hormones are produced and secreted into the blood.
Thyroid hormones secrete 80% T4 and 20% T3. T4 is inactive thyroid hormone while T3 is active thyroid hormone. After T4 is secreted into the blood, it turns into the active T3 form. As the level of the active T3 hormone rises, TSH levels are also suppressed and reduced. This cycle continues, changing throughout the day. Enzymatic or functional disruptions in this cycle can cause many diseases.