TSH; thyrotropin (American spelling), thyrotrophin (British spelling)
Thyroid Stimulating Hormone (TSH) is produced and released into the bloodstream by the pituitary gland. It stimulates the production of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3), by the thyroid gland by binding to its receptors in the thyroid gland. Thyroxine (T4) and triiodothyronine (T3) are essential for maintaining the body’s metabolic rate, heart and digestive functions, muscle control, brain development and bone activity.
A gland in the brain called the hypothalamus produces a hormone called thyrotrophin-releasing hormone (TRH). Thyrotrophin is another name for Thyroid Stimulating Hormone (TSH), which is so called as ‘thyro’ refers to the thyroid gland and ‘troph’ means to nourish, stimulate or grow, thus referring to TSH’s action to stimulate the thyroid gland. Thus, the hypothalamus produces TRH which stimulates the pituitary gland to produce TSH, which in turn stimulates the thyroid gland to produce the thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Thyroxine (T4) is the more abundant thyroid hormone in the blood stream but has a weaker action than triiodothyronine (T3). T3 is the more active form and can be formed from T4 in local tissues by enzymes called ‘deiodinases’, enabling local tissues to regulate the local activity of thyroid hormones.
The hypothalamus and pituitary gland sense the amount of thyroid hormone in the body, and in response, regulate the amount of TRH released from the hypothalamus and TSH from the pituitary gland, to maintain that the appropriate amount of thyroid hormones.
For example, if too much thyroid hormone is produced, the hypothalamus will respond by making less TRH and the pituitary gland will make less TSH in order to not stimulate the thyroid gland as much, and help bring thyroid hormone levels back down to normal. TRH is not easily measured with blood tests, whereas TSH is and can therefore indicate whether the hypothalamus and pituitary gland believe that there is the right amount of thyroid hormones in the body.
Conversely, if there is a problem with the thyroid gland causing it to make too little thyroid hormone, then the pituitary gland will increase the amount of TSH it produces in order to stimulate the thyroid gland to increase thyroid hormone production. This is called a ‘negative feedback loop’, whereby the controlling gland senses the amount of hormone from the gland it controls, and regulates the secretion of the controlling (often stimulating) hormone (in this case TSH) to ensure that the thyroid gland makes the appropriate amount of the end hormone (T4 and T3 in this case).
If a person has low thyroid hormone levels, commonly referred to as an underactive thyroid gland, or hypothyroidism (commonly due to an autoimmune condition destroying the thyroid gland called Hashimoto’s thyroiditis), the pituitary gland will make more TSH to try to stimulate the thyroid gland to make more thyroid hormone. Thus, a high TSH is an appropriate reaction by the pituitary gland to the thyroid gland making too little thyroid hormone. People with an underactive thyroid gland have symptoms such as feeling lethargic, gaining weight and feeling the cold more than expected. Their thyroid gland may enlarge to produce a goitre. Treatment is medication in the form of thyroid hormone tablets (typically levothyroxine) to bring the level of thyroid hormones back to normal, which will in turn reduce the amount of thyroid stimulating hormone (TSH) in the blood. It is particularly important for pregnant women to have the correct amounts of thyroid stimulating hormone and thyroid hormones to ensure the healthy development of their babies. Thyroid stimulating hormone (TSH) is one of the hormones measured in new-borns, which may indirectly indicate low thyroid hormone levels if TSH is raised (high TSH and low thyroid hormones).
Less commonly, problems in the pituitary gland or rare genetic conditions can result in inappropriately high thyroid stimulating hormone (TSH), and high thyroid hormone levels. The pituitary gland can abnormally make too much Thyroid Stimulating Hormone (TSH) because there is a problem in the pituitary gland, eg a pituitary adenoma (benign tumour) that makes TSH, called a ‘TSHoma’. In this situation, the pituitary gland no longer responds appropriately to the amount of thyroid hormone in the body and produces excess TSH, which will overstimulate the thyroid gland causing it to produce excess thyroid hormones (high TSH and high thyroid hormones).
Most commonly a low Thyroid Stimulating Hormone (TSH) level is seen in the context of an overactive thyroid, or hyperthyroidism, whereby the thyroid gland is making too much thyroid hormone and the pituitary gland responds appropriately by making less TSH (low TSH and high thyroid hormones). People with an overactive thyroid have the opposite symptoms to those with hypothyroidism, i.e. they may lose weight (even if accounting for the amount they eat), feel abnormally hot and can experience palpitations (heart racing) or anxiety. They may also have a slightly enlarged thyroid gland (called goitre). The commonest cause is an autoimmune condition called Graves’ disease, which can be treated with medication in the form of tablets (e.g. carbimazole), which reduce the activity of the thyroid gland and return all thyroid hormone levels to normal. In some cases, radioactive iodine treatment or surgical removal of the thyroid gland may be needed.
Less commonly, decreased function of the pituitary gland (hypopituitarism) can occur for example due to a pituitary tumour or surgical treatment of the pituitary gland. In this situation, the pituitary gland makes an insufficient amount of TSH causing thyroid hormones to be low as a result (low TSH and low thyroid hormones).
Last reviewed: Nov 2021