Final answer:
T3 and T4 are thyroid hormones that travel in the blood bound mainly to thyroxine-binding globulins (TBGs), albumin, and prealbumin. Most of T4 is converted to the more potent T3 in target tissues. These hormones are essential for regulating metabolism and energy use in the body.
Step-by-step explanation:
The hormones thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3) are primarily released by the thyroid gland following stimulation by thyroid-stimulating hormone (TSH). In the bloodstream, the majority of T3 and T4 are not freely circulating but are bound to transport proteins. Specifically, these thyroid hormones are bound to thyroxine-binding globulins (TBGs), albumin, and prealbumin (transthyretin). T4 and T3 attached to these proteins are biologically inactive but serve as a reservoir; when blood levels of free hormones decline, more bound T3 and T4 are released into circulation to be taken up by target cells.
In the target tissues, T4 can be converted into the more active T3 through deiodination, the process of removing an iodine atom. This transformation is crucial as T3 constitutes the active form of the thyroid hormone that regulates basal metabolic rate, affects nearly every cell in the body, and is involved in energy production and glucose oxidation. The regulation of hormones and the system's feedback mechanisms ensure that hormone levels remain balanced to maintain normal physiological functions.