What are the thyroid and parathyroid glands?
Where is the thyroid found?
What does the thyroid do?
In order to make T3 and T4 the thyroid gland needs iodine, a substance found in the food we eat. T4 is called this because it contains four atoms of iodine. T3 contains three atoms of iodine. In the cells and tissues of the body most T4 is converted to T3. T3 is the more active hormone, it influences the activity of all the cells and tissues of your body.
The other hormone that the thyroid makes is called calcitonin. This helps to control the levels of calcium and phosphorus in the blood. These minerals are needed, among other things, to keep bones strong and healthy.
How does the thyroid work?
The activity of the thyroid is controlled by hormones produced by two parts of the brain, the hypothalamus and the pituitary. The hypothalamus receives input from the body about the state of many different bodily functions. When the hypothalamus senses levels of T3 and T4 are low, or that the body's metabolic rate is low, it releases a hormone called thyrotropin-releasing hormone (TRH). TRH travels to the pituitary via the connecting blood vessels. TRH stimulates the pituitary to secrete thyroid-stimulating hormone (TSH).
TSH is released from the pituitary into the bloodstream and travels to the thyroid gland. Here TSH causes cells within the thyroid to make more T3 and T4. T3 and T4 are then released into the bloodstream where they increase metabolic activity in the body's cells. High levels of T3 stop the hypothalamus and pituitary from secreting more of their hormones. In turn this stops the thyroid producing T3 and T4. This system ensures that T3 and T4 should only be made when their levels are too low.
Calcitonin is released by the thyroid gland if the amount of calcium in the bloodstream is high. Calcitonin decreases the amount of calcium and phosphorus in the blood. It does this by slowing the activity of cells found in bone, called osteoclasts. These cells cause calcium to be released as they 'clean' bone. Calcitonin also accelerates the amount of calcium and phosphorus taken up by bone. Calcitonin works with parathyroid hormone to regulate calcium levels (see below for full explanation).
Where are the parathyroid glands found?
What do the parathyroid glands do?
- It causes the release of calcium from bones.
- It causes calcium to be absorbed (taken up into the blood) from the intestine.
- It stops the kidneys from excreting (getting rid of) calcium in the urine.
- It causes the kidneys to excrete phosphate in the urine.
- It increases blood levels of magnesium.
How do the parathyroids work?
When the calcium level is high in the bloodstream, the thyroid gland releases calcitonin. Calcitonin slows down the activity of the osteoclasts found in bone. This decreases blood calcium levels. When calcium levels decrease, this stimulates the parathyroid gland to release parathyroid hormone. Parathyroid hormone encourages the normal process of bone breakdown (essential for maintenance and growth of the bone). This process of bone breakdown releases calcium into the bloodstream. These actions raise calcium levels and counteract the effects of calcitonin. By having two hormones with opposing actions, the level of calcium in the blood can be carefully regulated.
Parathyroid hormone also acts on the kidneys. Here it slows down the amount of calcium and magnesium filtered from the blood into the urine. Parathyroid hormone also stimulates the kidneys to make calcitriol, the active form of vitamin D. Calcitriol helps to increase the amount of calcium, magnesium and phosphorus absorbed from your intestines (guts) into the blood.