TSH is Released from the Pituitary Gland
Thyroid-stimulating hormone (TSH) is secreted by the anterior pituitary gland. This is a fundamental aspect of the hypothalamic-pituitary-thyroid axis.
Anatomical Source and Physiological Context
TSH is produced and released by specialized cells called thyrotrophs in the anterior pituitary gland 1.
The anterior pituitary system includes TSH among its key hormones, along with adrenal corticotrophic hormone, prolactin, growth hormone, and the gonadotropins 1.
These anterior pituitary hormones are secreted under the influence of hypothalamic trophic factors, specifically thyrotropin-releasing hormone (TRH) from the hypothalamus in the case of TSH 1, 2.
The Regulatory Axis
The hypothalamus produces TRH, which stimulates TSH secretion from the anterior pituitary 2.
TSH then acts on the thyroid gland to stimulate thyroid hormone synthesis and release 3, 4.
This creates a negative feedback loop where thyroid hormones inhibit both TRH synthesis in the hypothalamus and TSH synthesis/release from the pituitary 2.
Clinical Relevance
Central hypothyroidism occurs when there is inadequate TSH secretion due to pituitary disease (secondary) or inadequate hypothalamic CRH/TRH (tertiary) 3.
Pituitary adenomas, craniopharyngiomas, pituitary surgery, and cranial radiation can all impair TSH production, leading to insufficient thyroid hormone stimulation 3, 5.
TSH measurements are the primary screening test for thyroid dysfunction, with normal reference ranges typically between 0.45-4.5 mIU/L 6, 7, 8.
To directly answer the multiple-choice question: The correct answer is "Pituitary." The thyroid gland is the target organ that responds to TSH, not its source. The hypothalamus regulates TSH release but does not directly secrete it. "Endocrine" is too general a term, as it refers to the entire endocrine system rather than a specific gland 1.