Thyroid-stimulating hormone

Thyroid-stimulating hormone, alpha
Identifiers
Symbol	CGA
Alt. symbols	HCG, GPHa, GPHA1
Entrez	1081
HUGO	1885
OMIM	118850
RefSeq	NM_000735
UniProt	P01215
Other data
Locus	Chr. 6 q14-q21

Thyroid-stimulating hormone, beta
Identifiers
Symbol	TSHB
Entrez	7252
HUGO	12372
OMIM	188540
RefSeq	NM_000549
UniProt	P01222
Other data
Locus	Chr. 1 p13

Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, TSH, or hTSH for human 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. It is a glycoprotein hormone synthesized and secreted by thyrotrope cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid.

Physiology

The system of the thyroid hormones T₃ and T₄.

Hormone levels

See also: Hypothalamic–pituitary–thyroid axis

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.

TSH is secreted throughout life but particularly reaches high levels during the periods of rapid growth and development.

The hypothalamus, in the base of the brain, produces thyrotropin-releasing hormone (TRH). TRH stimulates the 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. 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 resulting in both circadian and ultradian rhythms of its serum concentrations.

Subunits

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

• The α (alpha) subunit (i.e., chorionic gonadotropin alpha) is nearly identical to that of human chorionic gonadotropin (hCG), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). The α subunit is thought to be the effector region responsible for stimulation of adenylate cyclase (involved the generation of cAMP). The α chain has a 92-amino acid sequence.
• The β (beta) subunit (TSHB) is unique to TSH, and therefore determines its receptor specificity. The β chain has a 118-amino acid sequence.

The TSH receptor

Main article: TSH receptor

The TSH receptor is found mainly on thyroid follicular cells. Stimulation of the receptor increases T₃ and T₄ production and secretion. Stimulating antibodies to this 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. This is also the mechanism of trophoblastic tumors increasing the production of thyroid hormones.

Applications

Diagnostics

Main article: Thyroid-stimulating hormone measurement

Further information: Thyroid function tests

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 0.4-4.5 µIU/mL. 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".

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.

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 pathology	TSH level	Thyroid hormone level	Disease causing conditions
Hypothalamus/pituitary	High	High	Benign tumor of the pituitary (adenoma) or thyroid hormone resistance
Hypothalamus/pituitary	Low	Low	Secondary hypothyroidism or "central" hypothyroidism
Hyperthyroidism	Low	High	Primary hyperthyroidism i.e. Graves' disease
Hypothyroidism	High	Low	Congenital hypothyroidism (cretinism), 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.

Monitoring

The therapeutic target range TSH level for patients on treatment ranges between 0.3 to 3.0 μIU/mL.

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. TSH measurements could not be applied any more, however, treatment would have to be continued.

For hyperthyroid patients, both TSH and T₄ are usually monitored. It must also be noted that 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.

Therapeutic

A synthetic drug called recombinant human TSH alpha (rhTSHα or simply rhTSH) or thyrotropin alfa (INN) is manufactured by Genzyme Corp under the trade name Thyrogen. It is used to treat thyroid cancer.

External links

• TSH at Lab Tests Online
• MedlinePlus Encyclopedia 003684
• Thyrotropin at the US National Library of Medicine Medical Subject Headings (MeSH)