The below mentioned article provides a short note on the Thyroid Gland.
The thyroid gland weighs about 25-30 gm. and consists of closely packed sacs (follicles) filled with proteinaceous colloid. The gland secretes the hormones thyroxine and triiodothyronine. Of a total of 50 mg of iodine in the body, about 10-15 mg are in thyroid.
Thyroxine and Triiodothyronine:
Chemistry:
1. Iodination of tyrosine’s in thyroglobulin occurs first in position 3 of the aromatic nucleus and then at position 5 forming monoiodotyrosine and diiodotyrosine, respectively.
2. Coupling of two molecules of diiodotyrosine (I2 Tyr.) then occurs within the thyroglobulin molecule to form tetraiodothyronine (Thyroxine).
3. Coupling of monoiodotyrosine (I Tys.) with diiodotyrosine (I2 Tyr.) also occurs to form triiodothyronine (T3).
Protein Bound Iodine (PBI) in Blood:
The term PBI in blood represents iodine present in thyroid hormones. The PBI values for normal adults are 3.5-7.5 mg/100 ml of plasma. PBI is a reliable measure of thyroxine content of plasma.
The vlaues for PBI in hypo- and hyperthyroidism are given below:
Functions:
1. In hypothyroid animals, the tissues show a low rate of oxygen consumption, the patient has a slow pulse, lowered vigour, obesity, blood cholesterol levels are increased, lipolysis and fatty acid liberation are decreased. In hyperthyroid states, reverse occurs.
2. Thyroxine causes increased intestinal glucose absorption. It increases glycogenolysis in liver and muscle. It promotes neoglucogenesis.
3. It increases RNA, amino acid transport and protein synthesis.
4. In hypothyroidism, there is marked increase in serum cholesterol and triglyceride, and phospholipid contents in blood are also increased. In hyperthyroidism, the serum cholesterol level is decreased.
5. In high concentration of thyroid hormones, calcium is mobilized from bone, there are losses of potassium and nitrogen in urine, and calcium and phosphorus increase in the urine.
6. High concentration of thyroid hormone causes uncoupled oxidative phosphorylation and increased swelling in the mitochondria. Such action results in the production of heat rather than storage of energy as ATP.
7. Thyroxine is essential for the conversion of β-carotene to vitamin A.