Final answer:
Triiodothyronine (T3) is the major biologically active thyroid hormone, while thyroxine (T4) is secreted in larger quantities but is less potent and often converted to T3 in target tissues.
Step-by-step explanation:
Thyroid hormones include thyroxine (T4) and triiodothyronine (T3). T4, although secreted in larger quantities by the thyroid gland, is less potent and often converted to T3 in target tissues. T3 is the major biologically active form and is the more potent thyroid hormone, affecting almost all tissues in the body.
The production of both T4 and T3 is regulated by thyroid stimulating hormone (TSH), which is produced by the anterior pituitary gland. In the bloodstream, protein-bound T3 and T4 provide a reserve of these hormones, ensuring a constant supply to the body's tissues, which convert T4 to T3 by the removal of an iodine atom.