The thyroid

The thyroid is a butterfly-shaped gland found at the front of the neck and just below the voice box (larynx). It has two halves, called lobes, which lie on either side of the windpipe (trachea). The lobes are connected by a small band of thyroid tissue known as the isthmus.

The role of the thyroid

The thyroid is part of the endocrine system, which is a group of glands that makes and controls the body’s hormones. Hormones are chemical messengers that help the body work properly.

The thyroid makes two hormones (T4 and T3) that control the speed of the body’s processes, such as heart rate, digestion, body temperature and weight. This speed is known as your metabolic rate. The thyroid also produces the hormone calcitonin, which plays a role in controlling the body’s calcium levels.

Cells in the thyroid

There are two main types of cells in the thyroid:

• follicular cells – produce and store the hormones T4 and T3, and make a protein called thyroglobulin (Tg)
• parafollicular cells (C-cells) – produce the hormone calcitonin.

Parathyroid glands

Behind the thyroid are four additional glands known as the parathyroid glands. These glands produce parathyroid hormone (PTH), which works with calcitonin to control the amount of calcium in the blood.

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The role of thyroid hormones

The hormones T4 (thyroxine) and T3 (tri-iodothyronine) are known as the thyroid hormones. To make these hormones, the thyroid needs iodine, which is found in a range of foods such as seafood and iodised salt.

T4 is the main hormone made by the thyroid, but it is converted by the liver and kidneys into T3, a much more powerful hormone. Most T3 is created when the  liver and kidneys convert T4 into T3, but the thyroid also makes small amounts.

To keep the body working properly, it is important that the thyroid makes the right amounts of T4 and T3. This is controlled by the pituitary gland, which is located at the base of the brain:

• If the levels of T4 and T3 drop below normal, the pituitary gland produces a hormone called thyroid-stimulating hormone (TSH). TSH prompts the thyroid to make and release more T4 and T3.
• If the levels of T4 and T3 are too high, the pituitary gland produces less TSH.

Changes in thyroid hormone levels affect your metabolism by slowing down or  speeding up the body’s processes:

Underactive thyroid (hypothyroidism) – If you don’t have enough thyroid hormones, your metabolism slows down. As a result, you may feel tired or depressed, and gain weight easily. Other symptoms may include difficulty concentrating, constipation, brittle and dry hair and skin, sluggishness and fatigue. In severe cases, heart problems could occur.

Overactive thyroid (hyperthyroidism) – If you have too many thyroid hormones, your metabolism speeds up. As a result, you may lose weight, have increased appetite, feel shaky and anxious, or have rapid, strong heartbeats (palpitations). Over time, untreated hyperthyroidism can result in loss of bone strength and problems with heart rhythm.