The thyroid gland

Position:

• in the neck in front of, and in close contact with, the trachea.
• It is slightly reddish, consisting of two lobes joined by a narrow isthmus, surrounded by connective tissue; it is a follicular gland (its cells form follicles that store the hormone).

Hormone: thyroxine (amino-acid hormone). Iodine is essential for its formation. Effects of thyroxine:

1. Stimulates physical and mental growth and development.
2. Controls the basal metabolic rate (BMR): it raises oxidation of food and heat production.
3. Increases the absorption of monosaccharides from the intestine.
4. Keeps the skin and hair healthy. Thyroxine production is controlled by TSH from the pituitary; high thyroxine in the blood inhibits TSH secretion (negative feedback).

Calcitonin: the thyroid also secretes calcitonin, which lowers blood calcium and prevents calcium from being withdrawn from the bones (stimulates deposition in the bones).