Conversion of t4 to t3 in liver?

Thyroxine (T4), the major secretory product of the thyroid gland, is converted to its active form, 3,5,3’-triiodothyronine (T3), in peripheral tissues, particularly in the liver and kidneys, through a process known as deiodination. Deiodination involves the removal of one iodine atom from the outer ring of T4 to form T3.

In the liver, the deiodination of T4 to T3 primarily occurs through the activity of the enzyme type 2 deiodinase (D2), which is located primarily in the endoplasmic reticulum of liver cells. D2 catalyzes the removal of the iodine atom at the 5’ position of the outer ring of T4.

Here's an overview of the process:

1. Uptake: T4 is taken up into the liver cells from the bloodstream.

2. Binding: Inside the liver cells, T4 binds to specific transport proteins, such as thyroxine-binding globulin (TBG) and transthyretin, which help stabilize and transport it within the cell.

3. Deiodination: D2 enzyme, located in the endoplasmic reticulum, catalyzes the removal of the iodine atom from the 5’ position of the outer ring of T4, converting it into T3.

4. Release: The newly formed T3 is released from the liver cells into the bloodstream, where it can exert its biological effects on target tissues throughout the body.

The conversion of T4 to T3 in the liver plays a crucial role in regulating the overall thyroid hormone activity and metabolism in the body.

In addition to D2, there are other deiodinase enzymes involved in the metabolism of thyroid hormones, such as type 1 deiodinase (D1) and type 3 deiodinase (D3), but their presence and activity in the liver are relatively less significant compared to D2.