What is the thyroid gland?

The thyroid gland (glandula thyreoidea), which is situated at the front of the throat, is made up of two sections (lobes) that are connected to one another in the middle. Because of this, the thyroid gland looks a bit like a butterfly. The narrow piece that connects the two 'wings' is located on top of the windpipe, or trachea. There are also four parathyroid glands; two behind each lobe.

The thyroid gland is formed relatively early during pregnancy, approximately three weeks after fertilization. When the fetus is eleven weeks old, the thyroid gland is already working, but a lot had to happen before that point to make this possible. The first thyroid cells are formed in the oral cavity, from the same tissue that later becomes the tongue. The developing thyroid gland then travels down a tube, the thyroglossal duct, to its final location at the front of the throat. During this journey, the number of cells that make up the thyroid gland grows, and the underside of the growing gland splits into two parts. This results in a left and a right lobe, which remain connected in the middle. Once the thyroid gland is in its final location, it develops into a working thyroid gland and the thyroglossal duct closes of its own accord.

What does the thyroid gland do?

The thyroid gland consists of follicular cells and C cells. The follicular cells of the thyroid gland together form the wall of a follicle (small cavity), which is filled with a protein-rich liquid. The follicular cells produce hormones, mainly thyroxine (T4) and a small amount of triiodothyronine (T3). Both hormones are bound to the thyroglobulin (Tg) protein, which is stored in the thyroid follicles.

The pituitary, an endocrine gland in the brain, controls the thyroid gland and, in doing so, ensures that the hormones produced end up in the bloodstream. By way of the bloodstream, the thyroid hormones reach all of the cells in all parts of the body. Thyroid hormone plays a role in the metabolism of virtually all bodily tissues, and it stimulates longitudinal growth of the bones. Before birth and during the first three years of life, thyroid hormone is important for the growth and development of the brain and, after that, for its function.
The C cells produce a different hormone called calcitonin. Among other things, calcitonin inhibits the breakdown of bone tissue and ensures that calcium and other salts are excreted via the kidneys.

What do the parathyroid glands do?

The parathyroid glands are small glands which produce and secrete parathyroid hormone (PTH). Magnesium is needed for the adequate production of PTH. PTH has an influence on the amount of calcium and phosphate in the blood. It makes sure that your blood contains sufficient calcium. It does this in the following ways:

• PTH breaks down the bone and increases the resorption (breakdown) of bone cells, a process which releases calcium and phosphate into the blood;
• PTH ensures that less calcium and more phosphate are excreted by the kidneys;
• PTH stimulates the production of vitamin D, which increases the uptake of calcium by the intestines.