Does the hypothalamus secrete Thyrotropin-Releasing Hormone (TRH) through the anterior or posterior pituitary?

From the Guidelines

The hypothalamus secretes TRH into the hypophyseal portal system, which carries it to the anterior pituitary, where it stimulates the production and release of thyroid-stimulating hormone (TSH). This process is crucial for the regulation of thyroid hormones in the body. The hypothalamic-pituitary axis plays a vital role in this regulation, with the hypothalamus producing releasing hormones like TRH, which act on the anterior pituitary to stimulate the production of TSH [ 1 ]. The anterior pituitary then secretes TSH into the general circulation, which in turn stimulates the thyroid gland to produce thyroid hormones T3 and T4, regulating metabolism throughout the body. Key points to consider include:

• The hypothalamus produces TRH, which is then transported to the anterior pituitary via the hypophyseal portal system [ 1 ].
• The anterior pituitary responds to TRH by producing and releasing TSH [ 1 ].
• TSH stimulates the thyroid gland to produce thyroid hormones T3 and T4, which are essential for regulating metabolism [ 1 ]. It is essential to understand the distinction between the roles of the hypothalamus and the anterior pituitary in the regulation of thyroid hormones, as this knowledge can inform the diagnosis and treatment of thyroid-related disorders [ 1 ].

From the Research

Hypothalamic TRH Secretion

• The hypothalamus secretes thyrotropin-releasing hormone (TRH) to stimulate the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary gland 2, 3, 4, 5.
• TRH is the main positive regulator of TSH secretion, and its action is integrated with the negative feedback of thyroid hormone to guarantee appropriate thyroid stimulation 2.
• The hypothalamic-pituitary-thyroid axis determines the set point of thyroid hormone production, with TRH stimulating the synthesis and secretion of TSH, which acts at the thyroid to stimulate all steps of thyroid hormone biosynthesis and secretion 3.

Anterior vs. Posterior Pituitary

• TRH stimulates TSH secretion from the anterior pituitary gland, not the posterior pituitary gland 4, 5.
• The anterior pituitary gland is responsible for the secretion of TSH, which is then released into the bloodstream to stimulate the thyroid gland 3, 4.
• There is no evidence to suggest that TRH is secreted through the posterior pituitary gland to regulate TSH secretion 2, 3, 4, 6, 5.

Regulation of the Hypothalamic-Pituitary-Thyroid Axis

• The hypothalamic-pituitary-thyroid axis is regulated by a complex mechanism of positive and negative regulation, with TRH and thyroid hormone playing critical roles 2, 3, 5.
• TRH is absolutely required for both TSH and thyroid hormone synthesis, but is not necessary for thyrotroph cell development 4.
• The diagnosis and treatment of central hypothyroidism are complicated by the interaction of the hypothalamic-pituitary-thyroid axis with other hypothalamic-pituitary-hormone axes 6.