Which statement is not true about thyroxine?
A. Iron is essential for the synthesis of thyroxine.
B. It regulates carbohydrates, protein and fat metabolism in the body.
C. Thyroid gland requires iodine to synthesize thyroxine.
D. Thyroxine is also called thyroid hormone.

Hint: Thyroxine is the amino acid derived hormone containing tyrosine as a precursor. Generally thyroid gland produced two types of hormone that are triiodothyronine ${T_3}$ and thyroxine ${T_4}$. The hormone is mainly responsible for the regulation of minerals like iodine in the body and also helps in maintaining the basal metabolic rate (BMR).

Complete Answer:
The thyroid gland is comprises of two lobes that are located on either side of the trachea dorsally. Both the lobes of trachea are interconnected by the help of thin flap of connective tissue i.e. called as isthmus.
The composition of thyroid gland is:
(I) Thyroid follicles
(II) Stromal tissues
- Each thyroid follicle of the thyroid gland is having follicular cells that are enclosed in a cavity and is responsible for the production of two types of hormone that are triiodothyronine ${T_3}$ and tetraiodothyronine or thyroxine ${T_4}$.
- Iodine acts as a precursor element i.e. responsible for the production of thyroxine hormone in the body.

The correct option is A i.e. Iron is essential for the synthesis of thyroxine.

Additional information:
Thyroid hormone plays an important role:
- In the regulation of basal metabolic rate (BMR) of the body.
- In the process of erythropoiesis (production of red blood cells in the body).
- Control the metabolism of carbohydrates, proteins and fats.
- Helps in osmoregulation (maintenance of electrolyte and water level in the body)
- Helps in the secretion of hormone i.e. proteinaceous in nature called thyrocalcitonin (TCT) that regulates blood calcium level in the body.

Note: The deficiency of iodine in the body leads to the deficiency of the thyroxine thus the condition is known as hypothyroidism and this causes the enlargement of the thyroid gland commonly called as goitre.