Thyroid Stimulating Hormone – TSH

Thyrotropin (Thyroid Stimulating Hormone)

Thyrotropin or Thyroid Stimulating Hormone is a Glycoprotein hormone.
TSH, FSH and LH are the glycoprotein hormones of the anterior pituitary.
All 3 are made up of two subunits – alpha unit – common to all, while the beta subunit is specific for each.
The alpha unit is also common to human chorionic gonadotropin (hCG); so beta hCG is estimated as an index of pregnancy (Beta hCG is a tumor marker for choriocarcinoma).
This hormone may be called as:
- Thyroid Stimulating Hormone or
- TSH or
- Thyrotropic Hormone or
- Thyrotropin.

It is a Glycoprotein hormone produced by basophil cells of anterior pituitary.
Its mol. wt. is approx. 30,000. It consists of α and β subunits.
The α-subunit of TSH, LH, hCG and FSH are nearly identical.
The biological specificity of thyrotropin is in β-subunit
The α-subunit – 92 amino acids while β-subunit has 112 (may vary in different books).
Both α and β have several disulfide bridges.

The carbohydrate content of TSH is about 21% & its α and β chains bear 2 & 1 oligosaccharide chains respectively, linked by N-glycosidic linkages to specific asparagine residues.
The chains are synthesized separately by separate structural genes and later undergo post-translation modifications & glycosylations separately.

TSH increases the secretion of thyroid hormones by stimulating all the steps of production of synthesis of thyroxine (Iodine uptake etc.).
It acts through cAMP by binding with a receptor on thyroid cell surface.
The TSH secretion is stimulated by TRH.
The TRH also stimulates prolactin secretion.
Whereas, the prolactin inhibitory factor (PIF) inhibits secretion of TSH to some extent.
The TSH secretion is also controlled by the level of thyroid hormones.
It enhances the release of stored thyroid hormones.
It increases DNA content, RNA and translation of proteins, cell size.
It stimulates glycolysis, TCA cycle, HMP shunt and phospholipid synthesis. Stimulation of last two does not involve cAMP.
It activates adipose tissue lipase to enhance the release of fatty acids (lipolysis).
High levels of TSH may occur due to primary hypothyroidism and lack of feedback control.

Primary hyperthyroidism
Secondary hypothyroidism (pituitary origin)
Tertiary hypothyroidism (hypothalamic)
Subclinical hyperthyroidism (e.g. toxic m/nodular goiter, exogenous thyroid hormone administration, autonomous thyroid hormone secretion) and Euthyroid sick syndrome.

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