Question

The molecular structure of thyroxine is made up of _______
A. 2 iodine molecules along with T3
B. 4 iodine molecules along with T3
C. 3 iodine molecules along with T3
D. 1 iodine molecule along with T3

Answer 1

Hint: The chemical name of thyroxine is tetraiodothyronine.
The T3 is known as triiodothyronine. Thyroid hormones are insoluble in water but lipophilic in nature.

Complete answer: The two major hormones that are released from the thyroid gland are triiodothyronine or T3 and thyroxine or T4.
The thyroid hormones are produced in the follicular cells of the thyroid gland.
Both of these two hormones are tyrosine-based hormones, which means tyrosine is the precursor molecule.
Iodine is most important in the production of thyroid hormones.
T3 and T4 are composed of iodine.
As the name suggests, T3 has only 3 iodine presents in its structure and T4 has 4 iodine molecules present in its structure.
 It means there is one iodine more in T4 than T3.

Additional information: The chemical name of Thyroxine is L-3,5,3',5'-tetraiodothyronine, and
1. The chemical name of triiodothyronine is L-3,5,3'-triiodothyronine.
2. The position of iodine is important for the naming of T3 and T4. Iodine is present at three or four positions on the aromatic rings in T3 and T4 respectively.
3. The numbering of the iodine molecules is important for chemical naming.
4. Other iodinated molecules are also generated having little or no biological activity and are known as reverse T3 (chemical name 3,3',5'-triiodothyronine).
5. A large amount of the T3 is formed by the deiodination of T4 in peripheral tissues, especially in the liver and kidney.
6. Deiodination of T4 also produces reverse T3, which has no known metabolic activity in the human body.

So, the correct answer to this question is option D. 1 iodine molecule along with T3.

Note: T3 is more biologically active than T4, but the amount of secretion of T4 is higher (almost 90 %) than T3. These hormones act on intracellular receptors and bind to a carrier protein of blood, called thyroxine-binding globulin or TBG.