The following points highlight the functions of various types of thyroid hormones.
1. Functions of T3 and T4:
T3 and T4 are secreted from chief thyroid follicular cells.
The principal functions are given below:
(i) On Carbohydrate Metabolism:
(a) Enhances uptake of glucose in GI tract.
(b) Promotes gluconeogenesis in liver by enhancing the activities of PEPCK and pyruvate carboxylase.
(c) Causes insulin breakdown and increases blood glucose level.
(ii) On Protein Metabolism:
(a) Thyroid hormone has a specific effect on the tissues to mobilize protein and thereby release amino acids into extracellular fluids.
(b) It has anabolic effect, increase in RNA and protein synthesis.
(c) In hyperthyroidism, there is excessive proteins catabolism leading to body wasting as evidenced by excessive nitrogen excretion in the urine.
(iii) On Lipid Metabolism:
(a) The concentration of cholesterol in blood is inversely proportional with the level of thyroid activity.
(b) Thyroid hormone stimulates the synthesis, metabolism and degradation of lipids.
(c) It increases the oxidation of fatty acids.
(iv) On Vitamin Metabolism:
(a) It is necessary for conversion of carotene to Vit. A and Vit. A to retinol.
(b) Hyperthyroidism may cause Vit. D deficiency.
(c) In increases the requirement of Vit. B6, B12 and Vit C.
(v) On Iodine Metabolism:
Helps in the absorption of inorganic iodide from plasma and helps in oxidation of iodide into iodine for synthesis of thyroxine.
(vi) On Mineral Metabolism:
(a) It causes increased metabolism of calcium and phosphorous from the bone.
(b) Magnesium level in serum is increased in hyperthyroidism.
(vii) On Growth:
(a) It helps in muscular growth by synthesis of phosphocreatine.
(b) It plays an essential role in mental growth and development of central nervous system. Deficiency in young causes decrease of myelin, neuron size and axon number.
(c) It helps in tissue differentiation and maturation.
(viii) On Nervous System:
(a) Helps in development of nervous system, deficiency may cause failure of development of brain faculties.
(b) Deficiency may disturb reflex action.
(ix) On Reproductive System:
(a) It controls the normal function of gonads and development of secondary sex characters.
(b) Less amount of this hormone secretion delays onset of puberty.
(x) On Temperature Regulation:
It maintains the regulation of normal body temperature. Hypothyroidism increases susceptibility to moderate cold.
(xi) Calorigenic Effect:
It is calorigenic, heat production is required for the creation of energy in the form of ATP required for growth.
(xii) On Other Activities:
(a) It increases the tolerance of some drugs like morphine, digitalis etc.
(b) It increases the activities of different enzymes like phosphatase, cytochrome oxidase, hexokinase etc.
(c) It controls the blood pressure by controlling the heartbeat.
(d) It controls the rate and depth of respiration.
(e) It stimulates galactopoiesis (stimulates secretion of milk).
(f) It causes increase of urinary output.
(g) It increases the number and size of mitochondria.
2. Functions of Thyrocalcitonin (Calcitonin):
Thyrocalcitonin (TCT) or calcitonin (CT) is secreted from the Para follicular cells or c-cells of thyroid gland.
CT is a peptide hormone. It consists of 32 amino acid residues. Its molecular weight is about 3.6 KD. A disulfide linkage in between cys1 and cys7.
CT is continuously secreted. The secretion depends on Ca++. High Ca++ in serum may increase the secretion of CT, while less concentration of Ca++ reduces the CT secretion.
The function of CT is given below:
(i) Effects on Renal Tubule:
(a) It reduces the reabsorption of Ca++ and Pi from Henle’s loop and distal, convoluted tubule.
(b) It enhances Ca++ loss through urine.
(ii) On Bones:
(a) Promotes mineralization of bones.
(b) Prevents mobilization of Ca++ and Pi from bones to blood.
(c) It prevents Ca++ loss during pregnancy.
(d) It stimulates the growth of osteoblasts.
(iii) On Vit. D Metabolism:
Vit. D3 in the liver is converted to 25-hydroxycholecalciferol. CT in lung and kidney converts 25 hydroxycholecalciferol to 24, 25 di-hydroxycholecalciferol in presence of Vit. D metabolite which may influence Ca++ metabolism.
(iv) On Cyclic AMP Activity:
CT activates the adenylate cyclase cAMP system to raise their cytoplasmic concentration of cAMP which acts as the second messenger to produce the cellular effects.
(v) On Digestive System:
It involves in regulation of appetite.