Inside the thyroid gland, there are units called follicles, each of which works like a separate factory. Each follicle unit consists of follicular cells arranged side by side (thyrocytes) and a "colloid" in the middle space, which is used as a hormone reservoir. Among the follicular cells, there are "parafollicular" cells that secrete calcitonin. Thyroid medullary cancers develop from these cells.
The synthesis of thyroid hormones is made within the thyroid cell. Iodine taken from the diet enters the thyroid cell and produces T3 and T4 hormones. There are 3 iodines (tri-iodothyronine) in the T3 hormone, and 4 iodine (tetra-iodothyronine) in the T4 hormone. 80% of the hormones are found as T4, 20%as T3.
The hormone that enters the cell is T3, T4 does not enter the cell. Therefore, when there is a need for hormones, the body converts the T4 hormone into T3 to maintain a certain balance. T3 and T4 hormones circulate in the blood bound to protein. A small portion of these hormones are found in free form (Free T3 and Free T4). Measurement of free T3 and free T4 hormones is more valuable in dysfunctions of the thyroid gland.
The balance of thyroid hormones is regulated by the TSH hormone secreted from the pituitary gland in the brain. There is a negative balance between them. When the synthesis of thyroid hormones increases, TSH decreases; conversely, when the synthesis of thyroid hormones decreases, TSH increases.
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