Thyroid Stimulating Hormone (TSH) is a glycoprotein hormone secreted by the anterior pituitary gland that regulates thyroid gland function and thyroid hormone synthesis. It serves as the primary screening biomarker for thyroid dysfunction due to its high sensitivity in detecting even subtle changes in circulating thyroid hormone levels. TSH is measured via immunoassay and is considered the single most reliable indicator of thyroid axis status in ambulatory patients. TSH is released from thyrotroph cells of the anterior pituitary in response to thyrotropin-releasing hormone (TRH) from the hypothalamus, forming the hypothalamic-pituitary-thyroid (HPT) axis. Through a classic negative feedback loop, elevated free thyroxine (fT4) and triiodothyronine (fT3) suppress TSH secretion, while low thyroid hormone levels stimulate TSH release. TSH binds to receptors on thyroid follicular cells, stimulating the synthesis and secretion of T4 and T3, as well as thyroid gland growth.
Thyroid-stimulating hormone (TSH) is secreted by the anterior pituitary and regulates thyroid gland output of T4 and T3. It is the primary screening marker for thyroid dysfunction due to its high sensitivity to even small changes in circulating thyroid hormone levels.
Elevated TSH indicates the pituitary is compensating for insufficient thyroid hormone production (primary hypothyroidism). Suppressed TSH suggests excess circulating thyroid hormone, as seen in hyperthyroidism or exogenous thyroid hormone use. Subclinical thyroid disease — where TSH is abnormal but T3/T4 remain in range — is common and often clinically significant.
TSH alone may not capture the full picture in individuals under high physiological stress. Non-thyroidal illness syndrome (euthyroid sick syndrome), which can occur with energy deficits or overtraining, may present with normal TSH but reduced T3 conversion. Pairing TSH with free T3 and free T4 provides a more complete thyroid assessment in active populations.
Turnaround Time
3 days (up to 10 days)
Fasting Required
No
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