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Thyroid Function Tests

Thyrotropin releasing hormone (TRH), a hypothalamic peptide, stimulates the anterior pituitary to synthesize and secrete thyroid stimulating hormone (TSH). TSH stimulates the release of triiodothyronine (T3) and (thyroxine) T4 from thyroglobulin and their secretion by the thyroid. T4 and T3 are secreted in a ratio of 10:1. Most circulating T3 and T4 are protein bound. Only the free fraction is metabolically active. In peripheral tissue, T4 is converted to T3, which is the active hormone that migrates into the target cell nuclei and binds to DNA.

Primary hypothyroidism is the most common thyroid disorder. In this condition, the thyroid fails to respond to TSH. The pituitary secretes more TSH in response to decreasing circulating T4 concentrations. The failing thyroid gland partially compensates by increasing the ratio of secreted T3 to T4. If this last attempt to restore homeostasis fails, the patient’s metabolic rate falls. At this stage of hypothyroidism, TSH is elevated

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