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Thyroid stimulating hormone (TSH) is a glycoprotein hormone of 28KD secreted by the anterior pituitary gland. TSH has two subunits, namely α and β. The α subunit of TSH is similar to the α subunit found in the LH, FSH and hCG glycoprotein hormones. The β subunit however, is speciﬁ c and differs from hormone to hormone.
TSH stimulates positively the production of thyroid hormones T4 and T3. Circulating T4 and T3 regulate the TSH secretion by negative feedback. TSH production is also under the positive control of thyrotropin-releasing hormone (TRH), which is secreted by hypothalamus.
Measurement of serum TSH is generally regarded as the most sensitive indicator available for the diagnosis of primary and secondary hypothyroidism. In primary hypothyroidism, where there is impaired production of thyroid hormones, the TSH level is typically highly elevated. In secondary or tertiary hypothyroidism where the thyroid hormones are low as a consequence of pituitary or hypothalamic lesions, the TSH level is usually low.
Further, a sensitive TSH assay is also able to differentiate the hyperthyroidism from the euthyroid population. TSH is typically suppressed to subnormal levels in most hyperthyroidism.
It is recommended to assay both TSH and thyroid hormones for the clinical assessment of thyroid status. But if there were to be only one test to be prescribed for thyroid function, TSH would be the test. TSH determinations are also helpful to monitor patients who receive thyroxine replacement therapy.