Pituitary function

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Last modified 01/03/2015

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Pituitary function

The pituitary gland

Pituitary function is regulated by the hypothalamus, to which it is connected via the pituitary stalk, which comprises portal blood capillaries and nerve fibres. The anterior pituitary is influenced by a variety of stimulatory and inhibitory hormones through these capillaries. The posterior pituitary is a collection of specialized nerve endings that derive from the hypothalamus.

Anterior pituitary hormones

TSH (thyroid-stimulating hormone), acts specifically on the thyroid gland to elicit secretion of thyroid hormones.

ACTH (adrenocorticotrophic hormone) acts specifically on the adrenal cortex to elicit secretion of cortisol.

LH (luteinizing hormone) and FSH (follicle-stimulating hormone), known jointly as the gonadotrophins, act cooperatively on the ovaries in women and the testes in men to stimulate sex hormone secretion and reproductive processes.

GH (growth hormone) acts directly on many tissues to modulate metabolism. Metabolic fuels (e.g. glucose, free fatty acids) in turn modify GH secretion.

Prolactin acts directly on the mammary glands to control lactation. Gonadal function is impaired by elevated circulating prolactin concentrations.

The hypothalamic factors that control anterior pituitary hormone secretion are shown in Figure 42.1.

Fig 42.1 Hypothalamic factors that regulate anterior pituitary functions.

Hyperprolactinaemia

Hyperprolactinaemia is common and can cause infertility in both sexes. An early indication in women is amenorrhoea and galactorrhoea, whereas in men there may be no early signs and the first indication of the presence of a prolactinoma may be when a large growing tumour begins to interfere with the optic nerves. Causes of hyperprolactinaemia include:

stress (venepuncture is sufficient to raise plasma prolactin in some patients)

drugs (e.g. oestrogens, phenothiazines, metoclopramide, α-methyl dopa)

seizures (acutely)

primary hypothyroidism (prolactin is stimulated by the raised TRH)

other pituitary disease.

If these causes are excluded, the differential diagnosis is between:

a prolactinoma (a prolactin-secreting pituitary tumour, commonly a microadenoma)

idiopathic hypersecretion, which may be due to impaired secretion of dopamine, the hypothalamic factor that inhibits prolactin release.

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