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Thyroid stimulating hormone (TSH),or thyrotropin, is a primary regulator of the functional state of the thyroid gland. Its production and release is stimulated by the hypothalamic thyrotropin-releasing hormone (TRH) and is controlled by levels of the thyroid hormones (thyroxine and triodothyronine) at the pituitary gland and possibly the hypothalamus. Serum TSH levels are raised in cases of primary hypothyroidism. The diagnosis of hypothyroidism is made by finding a low total or free T4 value and is confirmed by a raised TSH level. Mild primary hypothyroidism may be more difficult to diagnose by just measuring the level of total and free T4, because the total and free T4 value can sometimes be within the normal range. In these cases,TSH assays are useful for diagnosis since the levels of TSH are raised. ln hyperthyroidism, levels of T3 and T4 are raised and TSH level is reduced.