Overview

The thyroid gland uses iodine from food to make two thyroid hormones: triiodothyronine (T3) and thyroxine (T4). It also stores these thyroid hormones and releases them as they are needed. The hypothalamus and the pituitary gland, which are located in the brain, help control the thyroid gland. The hypothalamus releases thyrotropin-releasing hormone (TRH). This stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). When the hypothalamus and pituitary are working normally, they sense when:

• Thyroid hormone levels are low, so they secrete more TRH and TSH. This stimulates the thyroid to make more hormones.
• Thyroid hormone levels are too high, so they secrete less TRH and TSH. This reduces hormone production by the thyroid.

Disease or tumours of the pituitary gland can affect this process.

How do thyroid hormones work?

Thyroid hormones affect every cell and all the organs of the body. Too much thyroid hormone speeds things up and too little thyroid hormone slows things down. They:

• Control the rate at which your body burns calories (your metabolism). This affects whether you gain or lose weight.
• Can slow down or speed up your heartbeat.
• Can raise or lower your body temperature.
• Change how fast food moves through your digestive tract.
• Affect muscle strength.
• Control how quickly your body replaces dying cells.

Thyroid hormones are made by the thyroid gland. The thyroid gland makes and releases two thyroid hormones: thyroxine (T4) and triiodothyronine (T3).

The thyroid gland and the pituitary gland work together. The pituitary gland (located near the base of the brain) makes, stores, and releases thyroid-stimulating hormone (TSH). When TSH is secreted by the pituitary gland, it causes the thyroid gland to release more T3 and T4. A high TSH level means there isn't enough thyroid hormone, and a low TSH level means there is too much.