Transport and conversion of thyroid hormones

The body consists of various organs like the brain, liver, kidney, heart, and others. All these organs work together and help in the smooth functioning of the body. The circulatory system acts as a road connecting every organ of our body and helps in the communication of organs. T4 accounts for most of the hormones released from the thyroid gland. T4 accounts for 90%, whereas T3 accounts for only 10% of thyroid hormone secretion.

In blood, there are various plasma proteins (secreted by the liver) that act as carriers of these hormones. Most of these hormones bind with the plasma protein and very little remains in free form. T4 (99.98%) binds more easily with these plasma proteins as compared to those of T3 (99.8%) and the remaining 0.2% (a free form of T3) binds with tissues. Protein-bound hormones are not available for action or metabolism. 70% of circulating thyroid hormones bind with Thyroid binding globulin (TBG) proteins, 20% of them binds with albumin, and 10% with transthyretin.

Importance of plasma protein binding in the transport of thyroid hormones:

Prevent acute fluctuation of free thyroid hormone.

Prevent extra metabolism of hormones.

Actions of thyroid hormones

The concentration of T3 hormone secretion might be very less, but it is responsible for most of the activities of the thyroid hormone. The T4 hormone serves as prohormones for T3. It is converted into T3 by the deiodination process in various tissues in the body. There are three types of deiodinase enzyme in the body for deiodination of T4:

D1 enzyme- It is present mostly in the liver and kidney. It removes one iodine from T4, thus forms T3 hormones. The liver and kidney are major sites of the deiodination of T4 hormones. After deiodination, the T3 hormones now move out of the liver and kidney and reach the target cells.

D2 enzymes- The brain and pituitary glands have a different mechanism to produce T3 hormones. They don’t take T3 from blood, rather they directly take T4 hormones and change it into T3 by using their D2 enzymes.

D3 enzymes- These types of enzymes are present in the placenta. These enzymes remove iodine from the inner ring of T4 and convert it into T3.So, T3 is produced in vast quantities in the peripheral region of the body as compared to its synthesis at the thyroid gland.

Transport across the membranes

The deiodinase involves the conversion of T4 to T3 present in the intracellular region. The action of thyroid hormones is intracellular, so are their actions. These two require iodothyronines for metabolism as well. For quite a while it was accepted that TH diffused actively over the membrane of the cell. Still, a study has made it progressively evident that cell transmembrane transport of TH is regulated by transporters. These carriers decide the accessibility of iodothyronines to the intracellular for action and metabolism and that the TH transport is energy-dependent.