The thyroid gland is located at the front of the neck just below the Adam's apple (larynx). It is butterfly-shaped and consists of two lobes located either side of the windpipe (trachea). A normal thyroid gland is not usually outwardly visible or able to be felt if finger pressure is applied to the neck.
The thyroid gland produces hormones that regulate the body's metabolic rate, growth and development. It plays a role in controlling heart, muscle and digestive function, brain development and bone maintenance. Its correct functioning depends on a good supply of iodine from the diet. Cells producing thyroid hormones are very specialised in extracting and absorbing iodine from the blood and incorporating it into the thyroid hormones.
The signal comes from a small gland located at the bottom of our brain called the pituitary gland. The pituitary gland produces and sends out a hormone called thyroid-stimulating hormone (TSH). TSH then tells the thyroid gland how much hormone to produce and secrete. TSH levels in your blood are rising and falling depending on your body’s needs, to produce more or less thyroid hormones.
The pituitary gland responds either directly to the thyroid hormones in the blood, but it also responds to signals from the hypothalamus, which sits above the pituitary gland as part of your brain. The hypothalamus releases its own hormone thyrotropin-releasing hormone (TRH). TRH in turn stimulates the release of TSH in the pituitary, which then signals to the thyroid gland.
This whole network is also referred to as the hypothalamic-pituitary-thyroid axis (HPT) and it adapts to metabolic changes and your body’s needs.
The thyroid gland produces thyroxine (referred to as T4), which is a relatively inactive prohormone, and the highly active hormone called triiodothyronine (referred to as T3). Collectively, thyroxine and triiodothyronine are referred to as the thyroid hormones. The thyroid gland produces just 20% of the highly active T3, and it mainly produces the prohormone T4, which constitutes about 80% of the secreted thyroid hormones. Once secreted by the thyroid, specific enzymes in other tissues like the liver or kidneys convert T4 into the active hormone T3 (which accounts for most of the T3 in the body).
In addition, there are other hormone-producing cells within the thyroid gland called C-cells. These cells produce calcitonin. Calcitonin plays a role in regulating calcium and phosphate levels in the blood, which is important for your maintain healthy bones.
Normally the thyroid gland produces the exact number of hormones needed to keep your body’s metabolism running and in balance. As described earlier, TSH secreted by the pituitary gland remains at a constant level in your blood circulation, but the level increases when the T4 levels fall and decrease when T4 levels in the blood rise. This hypothalamic-pituitary-thyroid feedback loop keeps the levels of T4 in your blood stable and reacts to small changes immediately.
However, there are several disorders associated with the thyroid gland with most problems concerning the production of thyroid hormones. Either the thyroid gland produces too much hormone (called hyperthyroidism), resulting in your body using energy more quickly than it should; or your thyroid doesn't produce enough hormone (called hypothyroidism), resulting in your body using energy more slowly than it should. Rarely cancer of the thyroid gland may develop.
Typical symptoms for hyperthyroidism are weight loss, fast (and sometimes irregular) heart rate, irritability/nervousness, muscle weakness and tremors, changes in menstrual periods, sleep problems, eye problems and heat sensitivity.
Symptoms for hypothyroidism include weight gain, slower heart rate, fatigue, menstrual period abnormalities, forgetfulness, dry skin and hair, hoarse voice and intolerance to cold.
In addition, both hyperthyroidism and hypothyroidism can be accompanied by an enlargement of the thyroid gland known as goitre.
On a worldwide scale, it is estimated that more than 200 million people have some form of thyroid disease. People of all ages and races can get thyroid disease. However, women are 5 to 10 times more likely than men to develop problems with their thyroid function.
There are different factors that can cause hyper- and hypothyroidism.
The following conditions cause hypothyroidism:
Thyroiditis is inflammation of the thyroid gland. This can reduce the amount of thyroid hormones produced.
A special form of thyroiditis is Hashimoto's thyroiditis. This is a genetic disorder that causes the immune system to make the thyroid gland underactive. Hashimoto’s thyroiditis often runs in families. In addition, thyroiditis can occur in women after giving birth, this is called postpartum thyroiditis. Postpartum thyroiditis is usually a temporary condition and occurs only in 5-9% of woman giving birth.
Nutrition also impacts the function of your thyroid gland. Iodine deficiency can cause hypothyroidism. This is a worldwide problem affecting approximately 100 million people. As mentioned earlier, iodine is used by the thyroid gland to produce hormones.
Medications, radioactive iodine treatment and thyroid surgery and conditions affecting the pituitary gland can also result in hypothyroidism.
The following conditions cause hyperthyroidism:
Graves' disease is a condition where the immune system causes the thyroid gland to become overactive and produce too much hormone. Your thyroid gland might be enlarged and referred to as a diffuse toxic goitre.
Thyroiditis (inflammation) caused by a viral infection can trigger the release of hormones that were stored in the thyroid gland. This uncontrolled release of thyroid hormones causes hyperthyroidism for a few weeks or months, with thyroid function returning to normal in most cases.
Excessive iodine intake may have negative effects on your thyroid gland. High quantities of iodine are found in a number of drugs such as Amiodarone, Lugol's solution (iodine), some cough syrups and contrast dyes used for some types of scans. This might cause the thyroid to produce either too much or too little thyroid hormones in some individuals.
Swelling and lumps can occur within the thyroid gland, and they are called nodules. Most thyroid nodules are harmless, but some can cause the overproduction of thyroid hormones. Rarely, thyroid nodules can be cancerous. In some cases, such as cancer, some or all of the thyroid gland is removed. You can live without your thyroid, but you need to take medicine daily to replace the hormones produced by your thyroid gland.
Iodine is the critical ‘ingredient’ for the production of thyroid hormones. We don’t need a lot of iodine, but a daily and constant supply of this micronutrient is important. Too much iodine can cause problems with your thyroid gland as described above. The best way to get your daily dose of iodine is through eating foods like seafood and dairy products. In addition, iodized salt (salt with iodine added to it) is a good source of iodine and you can use it to season your food.
Last reviewed: Jan 2020