Thyroid-stimulating hormone (TSH, thyrotropin) is a hormone produced by the anterior pituitary gland, which controls the thyroid hormones production.
Thyroid stimulating hormone is produced by the thyrotroph cells of the anterior pituitary gland. The hormone is a glycoprotein comprised of two subunits – α and β. The α-subunit is common to TSH, LH, FSH, and hCG, whereas each hormone has a unique β-subunit which represents the unique functions of TSH. The hormone is stored within the secretory granules in the hypophysis and released in response to thyreotropin-releasing hormone, which is secreted by the hypothalamus. In turn, TSH controls production of thyroid hormones (triiodothyronine – T3 and thyroxine – T4) responsible for the body’s metabolism by acting on the TSH receptors of the thyroid gland.
Every day, approximately 100 to 400 mU of TSH is produced. Its secretion has a pulsatile manner with peak concentrations occurring between 2 and 4 am and the lowest levels detected between 5 and 6 pm. TSH concentration rises up to 15-fold in case of hypothyroidism.
Functions of TSH
Thyroid stimulating hormone stimulates production of thyroid hormones by acting on the TSH G protein-coupled receptors.
- The hormone stimulates the growth and vascularity of the thyroid gland;
- Promotes thyroid hormone synthesis by boosting the uptake of iodine, organification of iodine onto tyrosine, coupling of tyrosines, and stimulate the release of stored thyroid hormone from thyroglobulin stores.
Concentration of thyroid stimulating hormone rise shortly after the birth of the baby with a surge of 25-160 mIU/l and decreases slowly until the third day of the baby’s life. Up to several weeks after birth TSH values in the blood of the baby reaches average adult concentrations.
Normal values of TSH in adults are generally in-between 0.4 and 4.5 mIU/l, although may be somewhat increased in individuals over 55 years.
Indications for measurement
Thyroid stimulating hormone values should be measured when any disorder of the thyroid gland, hyper- or hypofunction of the thyroid is suspected. Our thyroid test can be used to check whether your TSH levels are increased or not.
- Hyperthyroidism – condition characterized by the increased values of thyroid hormones in the blood. In turn, the hypothalamus and pituithary gland try to suppress the production of thyroid hormones by decreasing the TSH concentration in the blood;
- Pituitary (secondary) hypothyroidism – when the pituitary gland is affected by any tumor, trauma or due to surgical intervention or radiotherapy the gland cannot produce sufficient amounts of the TSH leading to the suppression of thyroid hormone production and is known as the secondary hypothyroidism;
- Nonthyroid illness (also known as eythyroid sick syndrome) – the thyroid function test results (including TSH values) are abnormal, although there is no dysfunction of the pituitary or the thyroid gland itself;
Some medications including L-thyroxine, glucocorticoids, dopamine and its agonists, levodopa, amiodarone, apomorphine, and pyridoxine are known to decrease TSH concentrations in blood.
- Congenital and primary hypothyroidism – in this case the thyroid gland fails to produce thyroid hormones. In response to the decreased concentration of these hormones in the blood the pituitary gland begins to synthesize huge amounts of TSH in order to regulate levels of T3 and T4;
- TSH-producing pituitary tumors – some pituitary tumors, although very rare, may secrete TSH;
- Pituitary resistance to thyroid hormone – sometimes the hypophysis is not able to recognize the thyroid hormones in the blood. Respectively, the gland produce more TSH to stimulate the thyroid gland, despite the fact that thyroid gland function is completely normal;
Drugs such as dopamine antagonists, chlorpromazine, haloperidol, iodine-containing drugs, and amiodarone may increase TSH values.