This allows a relatively tight degree of control of thyroid hormone diffusion into target cells in that the bound hormones will be released if circulating free levels decrease. Once in the bloodstream, greater than 99% of circulating thyroid hormones are bound to transport proteins such as albumin and T4-binding globulins less than 1% exists in free form. When stimulated by thyroid- stimulating hormone (TSH), the follicular cells transport the activated thyroid hormones from the colloid center into the bloodstream ( Fig. The DIT and monoiodotyrosine molecules form covalent bonds with one another to constitute the active forms of thyroid hormone tetraiodothyronine or thyroxine (T4), formed by two DITs and carrying four iodine molecules) and triiodothyronine (T3), formed by a DIT and monoiodotyrosine and carrying three iodine molecules). A tyrosine residue iodinated by a single iodine molecule leads to the formation of the molecule monoiodotyrosine, and that iodinated by two iodine molecules leads to diiodotyrosine (DIT). Colloid also contains the enzyme thyroid peroxidase (TPO), which catalyzes the next major step in thyroid hormone synthesis, which is the iodination of tyrosine residues on Tg.
įollicular cells actively transport iodide anion across the cell membrane from the bloodstream into the cytoplasm via the Na/I symporter membrane protein.The process is dependent upon the presence of iodine and is outlined below:
The synthesis of thyroid hormones is a complex multistep process that occurs within the thyroid follicle unit.