Thyroid-stimulating hormone is a glycoprotein released by the anterior pituitary in response to hypothalamic TRH.
Mechanism: Binds TSH receptors on thyroid follicular cells, triggering production of T4 and T3. The pituitary uses TSH as its feedback knob. High TSH means the pituitary thinks the thyroid is underperforming.
Lab range: 0.4–4.0 mIU/L is the standard reference, but optimal is widely debated. Many practitioners target 0.5–2.0 for symptomatic thyroid health.
Caveats: TSH alone is insufficient. Peripheral conversion of T4 → T3 happens at the tissue level, so a 'normal' TSH can coexist with low Free T3 and clinical hypothyroid symptoms. Always pair with fT4 + fT3 + rT3.