17.3 The Pituitary Gland and Hypothalamus

The hypothalamus and pituitary gland work together to regulate hormones that control growth, metabolism, reproduction, and water balance. They form the primary link between the nervous system and the endocrine system, translating neural signals into hormonal responses that maintain homeostasis throughout the body.

The Pituitary Gland and Hypothalamus

Hypothalamus-pituitary endocrine regulation

The hypothalamus sits in the brain and acts as the command center that connects the nervous and endocrine systems. It receives neural input from all over the body and converts those signals into hormonal instructions. The pituitary gland hangs just below the hypothalamus, connected by a stalk called the hypophyseal stalk (also called the infundibulum). This stalk contains both blood vessels and nerve fibers that allow the two structures to communicate directly.

The hypothalamus controls the pituitary by producing two categories of hormones:

• Releasing hormones (e.g., TRH, GHRH, CRH, GnRH) travel to the anterior pituitary and stimulate it to secrete specific hormones
• Inhibiting hormones (e.g., somatostatin, prolactin-inhibiting factor) prevent the anterior pituitary from secreting specific hormones, keeping levels in check

The pituitary gland has two structurally and functionally distinct lobes:

• The anterior lobe (adenohypophysis) manufactures and secretes six major hormones: GH, TSH, ACTH, FSH, LH, and PRL. Hypothalamic releasing and inhibiting hormones reach the anterior lobe through a specialized capillary network called the hypophyseal portal system, not through nerve fibers.
• The posterior lobe (neurohypophysis) does not make its own hormones. Instead, it stores and releases two hormones (ADH and oxytocin) that are actually produced by neurosecretory cells in the hypothalamus and transported down axons into the posterior lobe.

This distinction matters: the anterior lobe is controlled by hormones carried in blood, while the posterior lobe is controlled by direct neural connections.

Neuroendocrine Integration and Regulation

The hypothalamus and pituitary form the core of the neuroendocrine system, where nervous system input gets translated into endocrine output. This integration happens through organized regulatory pathways called hypothalamic-pituitary-target gland axes. Each axis controls a specific set of body functions.

Two major examples:

• Hypothalamic-pituitary-thyroid (HPT) axis: Hypothalamus releases TRH → anterior pituitary releases TSH → thyroid gland releases T3/T4 → regulates metabolism
• Hypothalamic-pituitary-adrenal (HPA) axis: Hypothalamus releases CRH → anterior pituitary releases ACTH → adrenal cortex releases cortisol → regulates stress response

Feedback loops are what keep these axes in balance:

• Negative feedback is the most common mechanism. When hormone levels from the target gland rise high enough, they signal back to the hypothalamus and/or pituitary to reduce further stimulation. For example, high cortisol levels inhibit both CRH and ACTH release, preventing overproduction.
• Positive feedback is rare but occurs in specific situations. During childbirth, oxytocin stimulates uterine contractions, which in turn stimulate more oxytocin release, amplifying the response until delivery occurs.

Posterior pituitary hormone functions

Remember that the posterior pituitary doesn't synthesize these hormones. Neurons in the hypothalamus produce them, and they're stored in axon terminals within the posterior lobe until a neural signal triggers their release.

Antidiuretic hormone (ADH), also called vasopressin, maintains water balance and blood pressure through two main actions:

• It increases water reabsorption in the collecting ducts of the kidneys, which concentrates the urine and conserves body water. When ADH is deficient, the kidneys can't concentrate urine properly, resulting in diabetes insipidus, a condition marked by excessive dilute urine output and intense thirst.
• It constricts blood vessels (hence the name "vasopressin"), which raises blood pressure. This helps maintain adequate perfusion to vital organs during dehydration or blood loss.

Oxytocin has key roles in reproduction and social behavior:

• It stimulates strong uterine contractions during labor and delivery. This is a classic positive feedback loop: contractions push the baby against the cervix, which triggers more oxytocin release, which drives stronger contractions.
• It promotes milk letdown during breastfeeding by causing myoepithelial cells around the mammary gland alveoli to contract, squeezing milk toward the nipple. Oxytocin doesn't produce the milk (that's prolactin's job); it moves the milk out.
• It plays a role in social bonding and maternal behavior, contributing to attachment between parent and infant as well as pair bonding in adults.

Anterior pituitary hormones and control

Each anterior pituitary hormone has a specific hypothalamic releasing (and sometimes inhibiting) hormone that controls it. The table below summarizes the six major hormones, then each is explained in more detail.

Growth hormone (GH) acts on bones, muscles, and other tissues to stimulate growth, cell reproduction, and protein synthesis. GH also promotes fat breakdown and raises blood glucose. Overproduction in children causes gigantism; in adults it causes acromegaly (enlargement of hands, feet, and facial features). Underproduction in children leads to pituitary dwarfism. GHRH from the hypothalamus stimulates GH release, while somatostatin inhibits it.

Thyroid-stimulating hormone (TSH) targets the thyroid gland, stimulating production and secretion of thyroid hormones (T3 and T4). These thyroid hormones regulate metabolic rate, growth, and development. TRH from the hypothalamus stimulates TSH release. Rising T3/T4 levels feed back to inhibit both TRH and TSH.

Adrenocorticotropic hormone (ACTH) targets the adrenal cortex, stimulating production of glucocorticoids, primarily cortisol. Cortisol regulates metabolism, suppresses immune responses, and helps the body cope with stress. Excess ACTH or cortisol leads to Cushing's syndrome (weight gain, moon face, muscle weakness). Deficient cortisol production characterizes Addison's disease (fatigue, low blood pressure, hyperpigmentation). CRH from the hypothalamus stimulates ACTH release.

Follicle-stimulating hormone (FSH) targets the gonads. In females, it stimulates ovarian follicle development and estrogen production. In males, it acts on Sertoli cells in the testes to support sperm production (spermatogenesis). GnRH from the hypothalamus stimulates FSH release.

Luteinizing hormone (LH) also targets the gonads. In females, a surge of LH triggers ovulation and stimulates the corpus luteum to produce estrogen and progesterone. In males, LH acts on Leydig cells in the testes to stimulate testosterone production. GnRH from the hypothalamus stimulates LH release. Note that FSH and LH are both classified as gonadotropins and are both controlled by the same releasing hormone (GnRH).

Prolactin (PRL) targets the mammary glands to stimulate milk production during lactation. PRL is unique among anterior pituitary hormones because its primary hypothalamic control is inhibitory: prolactin-inhibiting factor (PIF), which is dopamine, tonically suppresses PRL release. This means that if the hypothalamic connection is severed, PRL levels actually rise (unlike all the other anterior pituitary hormones, which would fall). Prolactin-releasing factor (PRF) can stimulate its release, but the dominant control is inhibition. Excess PRL causes hyperprolactinemia, which can lead to inappropriate milk production (galactorrhea) and disrupted reproductive cycles.