Thyroxine (T4) must be converted into triiodothyronine (T3) to become biologically active, and this conversion requires two specific enzymes called deiodinase type 1 and type 2. Changes in the genes that produce these enzymes can reduce the body's ability to activate thyroid hormone.

The thyroid hormone T4 is changed into a more active form called T3 in organs like the liver and kidneys through a biochemical process.

The body converts the thyroid hormone T4 into its active form, T3, using enzymes that require selenium as a component.

The thyroid hormone T4 can be converted into reverse T3, a molecule that binds to thyroid hormone receptors without activating them, which reduces the overall effect of active thyroid hormone on cells.

In people with thyrotoxicosis caused by taking thyroid hormone medication, the rise in active thyroid hormone (FT3) is smaller and less proportional to the increase in precursor hormone (FT4) than in people whose thyroid gland overproduces hormones, because of lower enzyme activity and lack of natural signaling from the thyroid.

A thyroid hormone byproduct called rT3 might block the active thyroid hormone from working properly in your brain cells. This blockage could lower the hormone's effectiveness, which might play a role in causing certain mental health and brain development issues.