Hormones. The thyroid gland secretes following hormones.
Thyroxine (tetraiodothyronine or T4)' and tri-iodothyronine or T3 are secreted by the thyroid follicular cells.
Calcitonin is secreted by the C-cells of the thyroid gland.
(i) Thyroxine (T4) and Tri-iodothyronine (T3) T4 and T3 contain four and three atoms of iodine.
T3 is secreted in smaller amounts (10%) but it is more active and several times more potent than T4
T is converted into T3 by removal of one iodine in the liver, kidneys and some other tissues.
T4 and T3 have similar effects on the target cells, they are generally considered together under the name, thyroid hormone (TH).
The thyroid gland is the only gland that stores its hormones in large quantity.
T4 and T3 are synthesised by attaching iodine to the aminoacid tyrosine.
Function of T3 & T4 -
(a) They regulate the metabolic rate of the body and thus maintain basal metabolic rate (BMR)
(b) They stimulate protein synthesis and, therefore, promote growth of the body tissues.
(c) They regulate the development of mental faculties.
(d) As they increase heat production, thus they maintain body temperature.
(e) They help in metamorphosis of tadpole into adult frog. If thyroid gland of the tadpole (larva) is removed, the larva fails to change into an adult.
(f) They increase action of neurotransmitters like adrenaline and noradrenaline.
(ii) Calcitonin (CT). It is secreted when calcium level is high in the blood. It then lowers the calcium level by suppressing release of calcium ions from the bones. Thus calcitonin has an action opposite to that of the parathy- roid hormone on calcium metabolism.