Thyroid-stimulating hormone

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Thyroid-stimulating hormone, alpha
Identifiers
Symbol CGA
Alt. symbolsHCG, GPHa, GPHA1
NCBI gene 1081
HGNC 1885
OMIM 118850
RefSeq NM_000735
UniProt P01215
Other data
Locus Chr. 6 q14-q21
Search for
Structures Swiss-model
Domains InterPro
Thyroid-stimulating hormone, beta
Identifiers
Symbol TSHB
NCBI gene 7252
HGNC 12372
OMIM 188540
RefSeq NM_000549
UniProt P01222
Other data
Locus Chr. 1 p13
Search for
Structures Swiss-model
Domains InterPro

Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism of almost every tissue in the body. [1] It is a glycoprotein hormone produced by thyrotrope cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid. [2] [3]

Contents

Physiology

The system of the thyroid hormones T3 and T4. Thyroid system.svg
The system of the thyroid hormones T3 and T4.

Hormone levels

TSH (with a half-life of about an hour) stimulates the thyroid gland to secrete the hormone thyroxine (T4), which has only a slight effect on metabolism. T4 is converted to triiodothyronine (T3), which is the active hormone that stimulates metabolism. About 80% of this conversion is in the liver and other organs, and 20% in the thyroid itself. [1]

TSH is secreted throughout life but particularly reaches high levels during the periods of rapid growth and development, as well as in response to stress.

The hypothalamus, in the base of the brain, produces thyrotropin-releasing hormone (TRH). TRH stimulates the anterior pituitary gland to produce TSH.

Somatostatin is also produced by the hypothalamus, and has an opposite effect on the pituitary production of TSH, decreasing or inhibiting its release.

The concentration of thyroid hormones (T3 and T4) in the blood regulates the pituitary release of TSH; when T3 and T4 concentrations are low, the production of TSH is increased, and, conversely, when T3 and T4 concentrations are high, TSH production is decreased. This is an example of a negative feedback loop. [5] Any inappropriateness of measured values, for instance a low-normal TSH together with a low-normal T4 may signal tertiary (central) disease and a TSH to TRH pathology. Elevated reverse T3 (RT3) together with low-normal TSH and low-normal T3, T4 values, which is regarded as indicative for euthyroid sick syndrome, may also have to be investigated for chronic subacute thyroiditis (SAT) with output of subpotent hormones. Absence of antibodies in patients with diagnoses of an autoimmune thyroid in their past would always be suspicious for development to SAT even in the presence of a normal TSH because there is no known recovery from autoimmunity.

For clinical interpretation of laboratory results it is important to acknowledge that TSH is released in a pulsatile manner [6] [7] [8] resulting in both circadian and ultradian rhythms of its serum concentrations. [9]

Subunits

TSH is a glycoprotein and consists of two subunits, the alpha and the beta subunit.

The TSH receptor

The TSH receptor is found mainly on thyroid follicular cells. [12] Stimulation of the receptor increases T3 and T4 production and secretion. This occurs through stimulation of six steps in thyroid hormone synthesis: (1) Up-regulating the activity of the sodium-iodide symporter (NIS) on the basolateral membrane of thyroid follicular cells, thereby increasing intracellular concentrations of iodine (iodine trapping). (2) Stimulating iodination of thyroglobulin in the follicular lumen, a precursor protein of thyroid hormone. (3) Stimulating the conjugation of iodinated tyrosine residues. This leads to the formation of thyroxine (T4) and triiodothyronine (T3) that remain attached to the thyroglobulin protein. (4) Increased endocytocis of the iodinated thyroglobulin protein across the apical membrane back into the follicular cell. (5) Stimulation of proteolysis of iodinated thyroglobulin to form free thyroxine (T4) and triiodothyronine (T3). (6) Secretion of thyroxine (T4) and triiodothyronine (T3) across the basolateral membrane of follicular cells to enter the circulation. This occurs by an unknown mechanism. [13]

Stimulating antibodies to the TSH receptor mimic TSH and cause Graves' disease. In addition, hCG shows some cross-reactivity to the TSH receptor and therefore can stimulate production of thyroid hormones. In pregnancy, prolonged high concentrations of hCG can produce a transient condition termed gestational hyperthyroidism. [14] This is also the mechanism of trophoblastic tumors increasing the production of thyroid hormones.[ citation needed ]

Applications

Diagnostics

Reference ranges for TSH may vary slightly, depending on the method of analysis, and do not necessarily equate to cut-offs for diagnosing thyroid dysfunction. In the UK, guidelines issued by the Association for Clinical Biochemistry suggest a reference range of 0.4–4.0 µIU/mL (or mIU/L). [15] The National Academy of Clinical Biochemistry (NACB) stated that it expected the reference range for adults to be reduced to 0.4–2.5 µIU/mL, because research had shown that adults with an initially measured TSH level of over 2.0 µIU/mL had "an increased odds ratio of developing hypothyroidism over the [following] 20 years, especially if thyroid antibodies were elevated". [16]

TSH concentrations in children are normally higher than in adults. In 2002, the NACB recommended age-related reference limits starting from about 1.3 to 19 µIU/mL for normal-term infants at birth, dropping to 0.6–10 µIU/mL at 10 weeks old, 0.4–7.0 µIU/mL at 14 months and gradually dropping during childhood and puberty to adult levels, 0.3–3.0 µIU/mL. [17] :Section 2

Diagnosis of disease

TSH concentrations are measured as part of a thyroid function test in patients suspected of having an excess (hyperthyroidism) or deficiency (hypothyroidism) of thyroid hormones. Interpretation of the results depends on both the TSH and T4 concentrations. In some situations measurement of T3 may also be useful.

Source of pathologyTSH levelThyroid hormone levelDisease causing conditions
Hypothalamus/pituitaryHighHighBenign tumor of the pituitary (adenoma) or thyroid hormone resistance
Hypothalamus/pituitaryLowLow Secondary hypothyroidism or "central" hypothyroidism
HyperthyroidismLowHigh Primary hyperthyroidism i.e. Graves' disease
HypothyroidismHighLow Congenital hypothyroidism, Primary hypothyroidism i.e. Hashimoto's thyroiditis

A TSH assay is now also the recommended screening tool for thyroid disease. Recent advances in increasing the sensitivity of the TSH assay make it a better screening tool than free T4. [3]

Monitoring

The therapeutic target range TSH level for patients on treatment ranges between 0.3 and 3.0 μIU/mL. [18]

For hypothyroid patients on thyroxine, measurement of TSH alone is generally considered sufficient. An increase in TSH above the normal range indicates under-replacement or poor compliance with therapy. A significant reduction in TSH suggests over-treatment. In both cases, a change in dose may be required. A low or low-normal TSH value may also signal pituitary disease in the absence of replacement.[ citation needed ]

For hyperthyroid patients, both TSH and T4 are usually monitored. In pregnancy, TSH measurements do not seem to be a good marker for the well-known association of maternal thyroid hormone availability with offspring neurocognitive development. [19]

TSH distribution progressively shifts toward higher concentrations with age. [20]

Difficulties with interpretation of TSH measurement

  • Heterophile antibodies (which include human anti-mouse antibodies (HAMA) and Rheumatoid Factor (RF)), which bind weakly to the test assay's animal antibodies, causing a higher (or less commonly lower) TSH result than the actual true TSH level. [21] [22] Although the standard lab assay panels are designed to remove moderate levels of heterophilic antibodies, these fail to remove higher antibody levels. "Dr. Baumann [from Mayo Clinic] and her colleagues found that 4.4 percent of the hundreds of samples she tested were affected by heterophile antibodies.........The hallmark of this condition is a discrepancy between TSH value and free T4 value, and most important between laboratory values and patient's conditions. Endocrinologists, in particular, should be on alert for this."
  • Macro-TSH - endogenous antibodies bind to TSH reducing its activity, so the pituitary gland would need to produce more TSH to obtain the same overall level of TSH activity. [23]
  • TSH Isomers - natural variations of the TSH molecule, which have lower activity, so the pituitary gland would need to produce more TSH to obtain the same overall level of TSH activity. [24] [25]
  • The same TSH concentration may have a different meaning whether it is used for diagnosis of thyroid dysfunction or for monitoring of substitution therapy with levothyroxine. Reasons for this lack of generalisation are Simpson's paradox [26] and the fact that the TSH-T3 shunt is disrupted in treated hypothyroidism, so that the shape of the relation between free T4 and TSH concentration is distorted. [27]

Therapeutic

Synthetic recombinant human TSH alpha (rhTSHα or simply rhTSH) or thyrotropin alfa (INN) is manufactured by Genzyme Corp under the trade name Thyrogen. [28] [29] It is used to manipulate endocrine function of thyroid-derived cells, as part of the diagnosis and treatment of thyroid cancer. [30] [31]

A Cochrane review compared treatments using recombinant human thyrotropin-aided radioactive iodine to radioactive iodine alone. [32] In this review it was found that the recombinant human thyrotropin-aided radioactive iodine appeared to lead to a greater of thyroid volume at the increased risk of hypothyroidism. [32] No conclusive data on changes in quality of life with either treatments were found. [32]

History

In 1916, Bennett M. Allen and Philip E. Smith found that the pituitary contained a thyrotropic substance. [33] The first standardised purification protocol for this thyrotropic hormone was described by Charles George Lambie and Victor Trikojus, working at the University of Sydney in 1937. [34]

Related Research Articles

<span class="mw-page-title-main">Hyperthyroidism</span> Thyroid gland disease that involves an overproduction of thyroid hormone

Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidism. Some, however, use the terms interchangeably. Signs and symptoms vary between people and may include irritability, muscle weakness, sleeping problems, a fast heartbeat, heat intolerance, diarrhea, enlargement of the thyroid, hand tremor, and weight loss. Symptoms are typically less severe in the elderly and during pregnancy. An uncommon but life-threatening complication is thyroid storm in which an event such as an infection results in worsening symptoms such as confusion and a high temperature; this often results in death. The opposite is hypothyroidism, when the thyroid gland does not make enough thyroid hormone.

<span class="mw-page-title-main">Thyroid</span> Endocrine gland in the neck; secretes hormones that influence metabolism

The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans, it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the isthmus (pl.: isthmi). The thyroid gland is a butterfly-shaped gland located in the neck below the Adam's apple. Microscopically, the functional unit of the thyroid gland is the spherical thyroid follicle, lined with follicular cells (thyrocytes), and occasional parafollicular cells that surround a lumen containing colloid. The thyroid gland secretes three hormones: the two thyroid hormones – triiodothyronine (T3) and thyroxine (T4) – and a peptide hormone, calcitonin. The thyroid hormones influence the metabolic rate and protein synthesis and growth and development in children. Calcitonin plays a role in calcium homeostasis. Secretion of the two thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is secreted from the anterior pituitary gland. TSH is regulated by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus.

<span class="mw-page-title-main">Graves' disease</span> Autoimmune endocrine disease

Graves' disease, also known as toxic diffuse goiter, is an autoimmune disease that affects the thyroid. It frequently results in and is the most common cause of hyperthyroidism. It also often results in an enlarged thyroid. Signs and symptoms of hyperthyroidism may include irritability, muscle weakness, sleeping problems, a fast heartbeat, poor tolerance of heat, diarrhea and unintentional weight loss. Other symptoms may include thickening of the skin on the shins, known as pretibial myxedema, and eye bulging, a condition caused by Graves' ophthalmopathy. About 25 to 30% of people with the condition develop eye problems.

<span class="mw-page-title-main">Hypothyroidism</span> Endocrine disease

Hypothyroidism is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormones. It can cause a number of symptoms, such as poor ability to tolerate cold, a feeling of tiredness, constipation, slow heart rate, depression, and weight gain. Occasionally there may be swelling of the front part of the neck due to goitre. Untreated cases of hypothyroidism during pregnancy can lead to delays in growth and intellectual development in the baby or congenital iodine deficiency syndrome.

<span class="mw-page-title-main">Iodothyronine deiodinase</span> Class of enzymes

Iodothyronine deiodinases (EC 1.21.99.4 and EC 1.21.99.3) are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. Thyroxine (T4), the precursor of 3,5,3'-triiodothyronine (T3) is transformed into T3 by deiodinase activity. T3, through binding a nuclear thyroid hormone receptor, influences the expression of genes in practically every vertebrate cell. Iodothyronine deiodinases are unusual in that these enzymes contain selenium, in the form of an otherwise rare amino acid selenocysteine.

<span class="mw-page-title-main">Triiodothyronine</span> Chemical compound

Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate.

<span class="mw-page-title-main">Levothyroxine</span> Thyroid hormone

Levothyroxine, also known as L-thyroxine, is a synthetic form of the thyroid hormone thyroxine (T4). It is used to treat thyroid hormone deficiency (hypothyroidism), including a severe form known as myxedema coma. It may also be used to treat and prevent certain types of thyroid tumors. It is not indicated for weight loss. Levothyroxine is taken orally (by mouth) or given by intravenous injection. Levothyroxine has a half-life of 7.5 days when taken daily, so about six weeks is required for it to reach a steady level in the blood.

<span class="mw-page-title-main">Thyroid disease</span> Medical condition

Thyroid disease is a medical condition that affects the function of the thyroid gland. The thyroid gland is located at the front of the neck and produces thyroid hormones that travel through the blood to help regulate many other organs, meaning that it is an endocrine organ. These hormones normally act in the body to regulate energy use, infant development, and childhood development.

<span class="mw-page-title-main">Toxic multinodular goitre</span> Medical condition

Toxic multinodular goiter (TMNG), also known as multinodular toxic goiter (MNTG), is an active multinodular goiter associated with hyperthyroidism.

Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. TFTs may be requested if a patient is thought to suffer from hyperthyroidism or hypothyroidism, or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy. It is also requested routinely in conditions linked to thyroid disease, such as atrial fibrillation and anxiety disorder.

Goitrogens are substances that disrupt the production of thyroid hormones. This triggers the pituitary to release thyroid-stimulating hormone (TSH), which then promotes the growth of thyroid tissue, eventually leading to goiter.

Desiccated thyroid, also known as thyroid extract, is thyroid gland that has been dried and powdered for medical use. It is used to treat hypothyroidism. It is less preferred than levothyroxine. It is taken by mouth. Maximal effects may take up to three weeks to occur.

<span class="mw-page-title-main">Thyrotropin receptor</span> Mammalian protein found in Homo sapiens

The thyrotropin receptor is a receptor that responds to thyroid-stimulating hormone and stimulates the production of thyroxine (T4) and triiodothyronine (T3). The TSH receptor is a member of the G protein-coupled receptor superfamily of integral membrane proteins and is coupled to the Gs protein.

<span class="mw-page-title-main">Hypothalamic–pituitary–thyroid axis</span> Part of the neuroendocrine system

The hypothalamic–pituitary–thyroid axis is part of the neuroendocrine system responsible for the regulation of metabolism and also responds to stress.

<span class="mw-page-title-main">Reverse triiodothyronine</span> Chemical compound

Reverse triiodothyronine (3,3′,5′-triiodothyronine, reverse T3, or rT3) is an isomer of triiodothyronine (3,5,3′ triiodothyronine, T3).

An antithyroid agent is a hormone inhibitor acting upon thyroid hormones.

Euthyroid sick syndrome (ESS) is a state of adaptation or dysregulation of thyrotropic feedback control wherein the levels of T3 and/or T4 are abnormal, but the thyroid gland does not appear to be dysfunctional. This condition may result from allostatic responses of hypothalamus-pituitary-thyroid feedback control, dyshomeostatic disorders, drug interferences, and impaired assay characteristics in critical illness.

<span class="mw-page-title-main">Thyroid hormones</span> Hormones produced by the thyroid gland

Thyroid hormones are any hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4). They are tyrosine-based hormones that are primarily responsible for regulation of metabolism. T3 and T4 are partially composed of iodine, derived from food. A deficiency of iodine leads to decreased production of T3 and T4, enlarges the thyroid tissue and will cause the disease known as simple goitre.

Thyroid disease in pregnancy can affect the health of the mother as well as the child before and after delivery. Thyroid disorders are prevalent in women of child-bearing age and for this reason commonly present as a pre-existing disease in pregnancy, or after childbirth. Uncorrected thyroid dysfunction in pregnancy has adverse effects on fetal and maternal well-being. The deleterious effects of thyroid dysfunction can also extend beyond pregnancy and delivery to affect neurointellectual development in the early life of the child. Due to an increase in thyroxine binding globulin, an increase in placental type 3 deioidinase and the placental transfer of maternal thyroxine to the fetus, the demand for thyroid hormones is increased during pregnancy. The necessary increase in thyroid hormone production is facilitated by high human chorionic gonadotropin (hCG) concentrations, which bind the TSH receptor and stimulate the maternal thyroid to increase maternal thyroid hormone concentrations by roughly 50%. If the necessary increase in thyroid function cannot be met, this may cause a previously unnoticed (mild) thyroid disorder to worsen and become evident as gestational thyroid disease. Currently, there is not enough evidence to suggest that screening for thyroid dysfunction is beneficial, especially since treatment thyroid hormone supplementation may come with a risk of overtreatment. After women give birth, about 5% develop postpartum thyroiditis which can occur up to nine months afterwards. This is characterized by a short period of hyperthyroidism followed by a period of hypothyroidism; 20–40% remain permanently hypothyroid.

Pulsatile secretion is a biochemical phenomenon observed in a wide variety of cell and tissue types, in which chemical products are secreted in a regular temporal pattern. The most common cellular products observed to be released in this manner are intercellular signaling molecules such as hormones or neurotransmitters. Examples of hormones that are secreted pulsatilely include insulin, thyrotropin, TRH, gonadotropin-releasing hormone (GnRH) and growth hormone (GH). In the nervous system, pulsatility is observed in oscillatory activity from central pattern generators. In the heart, pacemakers are able to work and secrete in a pulsatile manner. A pulsatile secretion pattern is critical to the function of many hormones in order to maintain the delicate homeostatic balance necessary for essential life processes, such as development and reproduction. Variations of the concentration in a certain frequency can be critical to hormone function, as evidenced by the case of GnRH agonists, which cause functional inhibition of the receptor for GnRH due to profound downregulation in response to constant (tonic) stimulation. Pulsatility may function to sensitize target tissues to the hormone of interest and upregulate receptors, leading to improved responses. This heightened response may have served to improve the animal's fitness in its environment and promote its evolutionary retention.

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