The synthesis of thyroid hormones

Inside the thyroid gland, there are units called "follicle", each of which works like a factory. Each follicular unit is composed of follicular cells (thyrocytes) lined up side by side and "the colloid" that is used as a hormone reservoir in the middle space. Among the follicular cells, there are "parafollicular" cells that secrete calcitonin. Thyroid medullary carcinoma is originated from these cells.

Thyroid hormones are synthesized in the thyroid cell. The iodine taken by feeding enters the thyroid cell and the thyroid hormones T3 and T4 are produced. There are 3 iodine (tri-iodothyronine) in T3 hormone and 4 iodine (tetra-iodothyronine) in T4 hormone. 80% of the hormones are found as T4, 20% as T3.

The hormone that enters the cell is T3, T4 cannot enter the cell. Therefore, when the hormone is needed, the body converts the T4 hormone to T3 to provide a certain balance. The thyroid hormones T3 and T4 circulate in the blood as bound to protein. A small amount of these hormones are in the free state (Free T3 and Free T4).  The measurement of free T3 and free T4 hormones is more important in functional disorders of the thyroid gland.

The balance of thyroid hormones is regulated by the TSH hormone secreted from the pituitary gland located in the brain. There is a negative balance between them. When the synthesis of thyroid hormones increases, TSH production decreases, vice versa, TSH production increases when the synthesis of thyroid hormones decreases.

Hormone Tests

TSH measurement is sufficient for screening of the thyroid functions.  If a thyroid disease is suspected, T3, T4, free T3, and free T4 hormones are examined as well as TSH. Depending on the characteristic of the thyroid disease, the patient is followed up by examining some of these hormones.

The upper limit of TSH in a healthy person is accepted as 4 mIU/L. In pregnant women, the upper limit of TSH is targeted as 2.5 mIU/L in the first trimester and as 3 mIU/L in the following months. The first test to be studied in women who cannot get pregnant is the TSH level.

The upper limit of TSH is accepted as 6 mIU / L in people between the age of 70-80 years, and the upper limit of TSH in people over 80 years is accepted as 7.5 mIU/L. Elderly people don't need thyroid hormone drugs within these values.