Thyroid hormones

Thyroid hormones are the hormones secreted by the thyroid glands and they are:

1. Tetraiodothyronine (T4) or thyroxine
2. Triiodothyronine (T3)

Chemistry and source :

Both T3 and T4 are iodine containing derivatives of amino acid tyrosine. T4 forms about 90% of the secretion whereas T3 is only about 10%. But the potency of T3 is four times more than that of T4.

Synthesis :

The first step in the synthesis of thyroid hormones is the organification of iodine.  Iodide is taken up, converted to iodine, and then condensed onto tyrosine residues which reside along the polypeptide backbone of a protein molecule called thyroglobulin.  This reaction results in either a mono-iodinated tyrosine (MIT) or di-iodinated tyrosine (DIT) being incorporated into thyroglobulin.  This newly formed iodothyroglobulin forms one of the most important constituents of the colloid material, present in the follicle of the thyroid unit.

The other synthetic reaction, that is closely linked to organification, is a coupling reaction, where iodotyrosine molecules are coupled together.  If two di-iodotyrosine molecules couple together, the result is the formation of thyroxin (T4).  If a di-iodotyrosine and a mono-iodotyrosine are coupled together, the result is the formation of tri-iodothyronine (T3).

From the perspective of the formation of thyroid hormone, the major coupling reaction is the di-iodotyrosine coupling to produce T4.  Although T3 is more biologically active than T4, the major production of T3 actually occurs outside of the thyroid gland.  The majority of T3 is produced by peripheral conversion from T4 in a deiodination reaction involving a specific enzyme which removes one iodine from the outer ring of T4.

The T3 and T4 released from the thyroid by proteolysis reach the bloodstream where they are bound to thyroid hormone binding proteins. The major thyroid hormone binding protein is thyroxin binding globulin (TBG) which accounts for about 75% of the bound hormone.

Metabolic roles:

1. Effect on Basal Metabolic Rate:

• Thyroxine increases the basal metabolic rate (BMR) by increasing the oxygen consumption of the tissues.

2. Effect on body temperature:

• Due to the increase in BMR, the heat production by the body is also increased.

3. Effect on protein metabolism::

• Thyroid hormones increase the synthesis of proteins by increasing the translation of RNA.

4. Effect on carbohydrate metabolism:

• They increase absorption of glucose in GI tract and enhance the glucose uptake by cells.

5. Effect on fat metabolism:

• Thyroxine decreases the storage of fat and increases free fatty acids in the blood.
• But also helps in decreasing the cholesterol level.

6. Effect on blood:

• Thyroxine increases the production of RBCs. It is the important factor erythropoiesis.

7. Effect on heart:

• Thyroxine increases the heart rate and increases the force of contraction of the heart.