To control the level of these hormones the hypothalamus and pituitary secrete compounds called releasing factors. It is the amount of TSH circulating in the blood stream that tells the thryoid gland how much thyroxine to secrete. In a very refined feedback mechanism between the hypothalamus, pituitary, and thyroid gland, the cells of the body get just the right amount of T4 and T3. Thyroxine circulates throughout the bloodstream and affects almost all organs. It plays a major role in controlling metabolism, and is needed for growth.
Primary hypothyroidism accounts for almost every case.
This cause, also known as autoimmune thyroiditis, occurs when the body makes antibodies against the thyroid gland. This effectively destroys part of it, so it has less thyroxine to secrete into the bloodstream. It is one of the most common causes of primary hypothyroidism. This cause of hypothyroidism can start early in life. Symptoms will appear when it progresses to the point that the reserve power of the thryoid gland is affected.