Thyroid-Stimulating Hormone (TSH)
Definition: An anterior pituitary hormone that stimulates the thyroid gland to produce and secrete thyroxine (T4) and triiodothyronine (T3). The most sensitive screening test for thyroid dysfunction.
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Operates on an inverse negative feedback loop with thyroid hormones:
- Reference Range: Typically 0.4 to 4.5 mIU/L.
- Elevated TSH (>4.5 mIU/L): Indicates primary hypothyroidism (the pituitary produces excess TSH to stimulate an underactive thyroid).
- Suppressed / Low TSH (<0.4 mIU/L): Indicates hyperthyroidism or thyrotoxicosis (excess circulating T3/T4 suppresses pituitary secretion).
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