Thyroid Gland: Thyroxine and Triiodothyronine

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.