Consider the following statements regarding thyroid hormones-

Thyroid Hormones physiology The thyroid secretes predominantly thyroxine (T4) and only a small amount of triiodothyronine (T3); approximately 85% of T3 in blood is produced from T4 by a family of monodeiodinase enzymes that are active in many tissues, including liver, muscle, hea and kidney. Selenium is an integral component of these monodeiodinases. T4 can be regarded as a prohormone, since it has a longer half-life in blood than T3 (approximately 1 week compared with approximately 18 hours), and binds and activates thyroid hormone receptors less effectively than T3. T4 can also be conveed to the inactive metabolite, reverse T3. T3 and T4 circulate in plasma almost entirely (> 99%) bound to transpo proteins, mainly thyroxine-binding globulin (TBG). It is the unbound or free hormones that diffuse into tissues and exe diverse metabolic actions. Some laboratories use assays that measure total T4 and T3 in plasma but it is increasingly common to measure free T4 and free T3free hormone measurements is that they are not influenced by changes in the concentration of binding proteins. For example, TBG levels are increased by oestrogen (such as in the combined oral contraceptive pill) and this will result in raised total T3 and T4, although free thyroid hormone levels are normal. Ref - davidson 20e p656