Pituitary-Thyroid Feedback Axis

Scenario

Readouts

Diagnostic pattern

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TSH (pituitary signal)–

T4 (thyroid hormone)–

Pituitary gland

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Thyroid gland

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How it works

The pituitary continuously senses circulating T4 and adjusts TSH output inversely: high T4 suppresses TSH, low T4 raises it. TSH then drives the thyroid to produce more T4, closing the loop.

Healthy — both glands respond correctly; TSH and T4 settle near their targets.
Primary hypothyroidism — the thyroid itself can't respond even to maximal TSH, so T4 stays low while TSH climbs as the pituitary keeps pushing harder.
Secondary hypothyroidism — the pituitary can't produce enough TSH, so a perfectly capable thyroid is under-stimulated and both TSH and T4 stay low.

Measuring TSH and T4 together — not either alone — is what lets a clinician tell which gland is malfunctioning from the resulting pattern.

TSH pulse (pituitary → thyroid) T4 pulse (thyroid → pituitary) Malfunctioning gland

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