Anterior Pituitary Hormones and Their Regulation

The anterior pituitary, or adenohypophysis, secretes the trophic hormones that govern much of the endocrine system: growth hormone, prolactin, thyroid-stimulating hormone, the gonadotropins, and adrenocorticotropic hormone. Each is produced by a distinct cell type and regulated by hypothalamic input and by feedback from the glands it controls.

Definition

Anterior pituitary hormones are the peptide and glycoprotein hormones secreted by the adenohypophysis (GH, PRL, TSH, FSH, LH, and ACTH), each released by a specific cell type under combined hypothalamic and peripheral-feedback control.

Scope

The topic covers the six classic anterior pituitary hormones, the cell types that make them, the hypothalamic factors that switch them on or off, and the negative-feedback signals that constrain their output. It treats anterior pituitary regulation as a physiology topic and not as clinical guidance.

Core questions

• Which cell types produce each anterior pituitary hormone?
• How do hypothalamic releasing and inhibiting factors set the secretion of each hormone?
• How does feedback from peripheral hormones regulate pituitary output?
• Why are growth hormone and prolactin regulated differently from the glycoprotein hormones?

Key concepts

• Growth hormone (GH)
• Prolactin (PRL)
• Thyroid-stimulating hormone (TSH)
• Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
• Adrenocorticotropic hormone (ACTH)
• Adenohypophyseal cell types (somatotrophs, lactotrophs, thyrotrophs, gonadotrophs, corticotrophs)
• Glycoprotein versus single-chain peptide hormones
• Dual hypothalamic and feedback control

Mechanisms

Five differentiated cell types of the adenohypophysis each secrete a characteristic hormone. Somatotrophs release growth hormone under the opposing control of GHRH and somatostatin; lactotrophs release prolactin under tonic inhibition by hypothalamic dopamine; thyrotrophs release TSH in response to TRH; gonadotrophs release FSH and LH in response to pulsatile GnRH; and corticotrophs release ACTH in response to CRH. TSH, FSH, and LH share a common alpha subunit and a hormone-specific beta subunit, marking them as the glycoprotein family. Output of each axis is restrained by negative feedback from the relevant peripheral hormone (for instance thyroid hormones on thyrotrophs, and cortisol on corticotrophs), so secretion reflects the balance between hypothalamic drive and feedback inhibition.

Clinical relevance

Anterior pituitary hormones regulate growth, metabolism, the stress response, lactation, and reproduction, so their physiology underlies the interpretation of endocrine function throughout the health sciences. Knowing which cell and which feedback loop governs each hormone is how the level of a disturbance within an axis is reasoned about. This entry describes physiology and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The regulatory logic summarised here rests on the characterisation of hypothalamic releasing hormones by Schally, Guillemin, and colleagues and on subsequent feedback-control studies of individual axes, consolidated in standard endocrinology and physiology texts. The pulsatile control of the gonadotropins is detailed in reviews of the GnRH pulse generator.

History

The anterior pituitary was long recognised as a master endocrine gland, but how it was itself controlled remained unclear until the mid-twentieth century. Geoffrey Harris's portal-vessel hypothesis and the subsequent isolation of hypothalamic releasing hormones explained the regulation of each adenohypophyseal cell type, while immunocytochemistry and cell biology defined the distinct trophic cell lineages.

Key figures

• Andrew V. Schally
• Roger Guillemin
• Shlomo Melmed
• Allan E. Herbison

Seminal works

• schally-1973

Frequently asked questions

How many hormones does the anterior pituitary secrete?

Six classic hormones: growth hormone, prolactin, thyroid-stimulating hormone, the two gonadotropins (FSH and LH), and adrenocorticotropic hormone, made by five distinct cell types.

Why is prolactin regulated mainly by inhibition?

Lactotrophs secrete prolactin spontaneously, so the hypothalamus holds them in check through tonic dopamine. Releasing dopamine's brake raises prolactin, which is why prolactin behaves differently from the other anterior pituitary hormones.