Central hypothyroidism

Central hypothyroidism is insufficient thyroid hormone production caused by inadequate stimulation from the hypothalamus or pituitary, despite a potentially functional thyroid gland.

Hypothalamic–pituitary–thyroid axis: The hormonal feedback system linking the hypothalamus, pituitary gland, and thyroid. Central hypothyroidism results when this signaling chain fails upstream of the thyroid.
Free thyroxine: The unbound fraction of thyroxine circulating in blood and available to tissues. A low free thyroxine level is central to biochemical diagnosis.
Hypopituitarism: Reduced secretion of one or more hormones by the pituitary gland. Pituitary hormone loss is a common setting for inadequate TSH stimulation.
Adrenal insufficiency: Inadequate production of cortisol by the adrenal glands. It can coexist with pituitary failure and should be assessed before thyroid replacement.
Thyrotropin-releasing hormone: A hypothalamic hormone that stimulates pituitary thyrotrophs to release thyroid-stimulating hormone. Insufficient hypothalamic release can deprive the pituitary of a key stimulus.
Triiodothyronine: The most biologically active thyroid hormone, produced largely by conversion of thyroxine in tissues. Its concentration may remain less reduced than thyroxine in some cases.
Pituitary adenoma: A usually benign tumor arising from cells of the pituitary gland. Adenomas can disrupt thyrotroph function or compress surrounding pituitary tissue.
Adrenal crisis: A life-threatening state of severe cortisol deficiency causing circulatory and metabolic instability. Starting thyroid hormone before treating unrecognized cortisol deficiency can precipitate crisis.
Thyroid-stimulating hormone: A pituitary hormone that stimulates thyroid growth and the production of thyroid hormones. Low or biologically weak TSH leaves the thyroid inadequately stimulated.
Thyroid function tests: Blood tests that assess thyroid hormone levels and the signals regulating thyroid function. Interpreting TSH alongside free thyroxine reveals the central pattern.

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