Final answer:
Thyroid hormones such as T4 and T3 are transported through the bloodstream attached to proteins like thyroxine-binding globulins. When needed, they are released and enter target cells by crossing the lipid bilayer, where they bind to intracellular receptors to regulate metabolism.
Step-by-step explanation:
Thyroid hormones are essential for regulating metabolism in the body. These hormones, primarily thyroxine (T4) and triiodothyronine (T3), are produced by the thyroid gland and are transported in the bloodstream to reach their target cells. In the bloodstream, a large portion of T3 and T4 is bound to thyroxine-binding globulins (TBGs), as well as other plasma proteins like albumin, which helps keep these hormones soluble and prevents their free diffusion into cells.
At the target cells, when the levels of T3 and T4 decrease, these bound hormones are released from the plasma proteins and can cross the lipid bilayer of the cell membranes. T3 is the more active form and is often created in target cells by converting T4 into T3 by removing an iodine atom. Inside the cell, T3 and T4 pass through the cell membranes and bind to intracellular receptors which regulate gene expression and stimulate metabolic activities.