Flashcards in Endocrinology: Pituitary Deck (10)
The anterior pituitary produces six main hormones:
3) GH (growth hormone)
4) Prolactin (PRL)
5) Luteinising hormone (LH)
6) Follicle Stimulating Hormone (FSH).
The posterior pituitary secretes oxytocin and vasopressin (ADH).
Growth hormone secretion by somatotroph cells is
pulsatile and regulated by its hypothalamic releasing hormone GHRH (stimulatory), somatostatin (inhibitory), IGF-1, circadian rhythm, stress and various medications.
Hypoglycaemia stimulates growth hormone
Growth hormone stimulates the production of
insulin-like growth factor (IGF-1) in the liver and is
associated with bone growth, protein synthesis and
the production of free fatty acids. In muscle it has an anti-insulin effect evidenced by gluconeogenesis.
GH deficiency or resistance
Growth hormone resistance or failure of growth
hormone production in children due to a pituitary or
hypothalamic defect results in IGF-1 deficiency and
An excess of growth hormone in children causes gigantism, with particular emphasis on limb
growth due to the effects on the cartilaginous epiphyses.
In adults an excess of growth hormone (acromegaly) is associated with bone growth, particularly the skull and mandible, and soft tissue proliferation with a characteristic increase in hand, finger and foot size.
Carpal tunnel syndrome may occur. The skin is
thickened and coarse. Patients with acromegaly will
have impaired glucose tolerance; 10% or more will be diabetic. Most cases of acromegaly are due to a pituitary tumour (99%); tumours of the lung, pancreas (islet cells) and gut can secrete GHRH.
The biochemical diagnosis of acromegaly is made
by the fi nding of a high serum IGF-1 level and the
failure of GH levels to suppress after glucose loading. In normal individuals the GH level should suppress in response to glucose excess.
Prolactin secretion is episodic and is regulated consequent to tonic inhibition by dopamine; stress, and oestrogens increase prolactin levels. It acts mainly on the breast where it initiates and maintains lactation.
Prolactinoma is the most common functioning
tumour of the anterior pituitary; hyperprolactinaemia
may be caused by a variety of pituitary disorders,
drugs, chronic renal failure and hypothyroidism.
Galactorrhoea and symptoms of oestrogen deficiency occur in women; in men, reduced libido and impotence.
In women, LH stimulates ovarian oestrogen and progesterone secretion, and is responsible for ovulation. FSH is responsible for the development of the ovarian follicle.
In men, LH stimulates testosterone production;
FSH stimulates maturation and growth of the
seminiferous tubule cells.
Oxytocin and vasopressin
Oxytocin stimulates uterine contraction during labour and smooth muscle contraction in the breast during breast feeding.
Vasopressin (ADH) together with the kidneys maintains osmotic homeostasis; in normal circumstances the serum osmolality is almost constant. Water homeostasis is maintained by a complex interaction that involves osmoreceptors (plasma sodium concentration), baro-receptors (volume), thirst, ADH, atrial natriuretic peptide (ANP) and angiotensin.
When plasma osmolality increases (>280 mmol/L) plasma ADH rises, increasing the permeability of renal collecting duct cells to water allowing its reabsorption.
Sodium and ADH
Hyponatraemia and hypernatraemia are usually
caused by abnormalities of water balance rather
than abnormalities of sodium balance. When there
is damage to the posterior pituitary, there is reduced ADH release with resulting inability of the renal tubules to conserve water. Diabetes insipidus results – with polyuria, polydipsia and a raised haematocrit due to haemoconcentration. In hospital patients hyponatraemia is most commonly caused by excess intravenous fluid administration, heart failure, cirrhosis, hypothyroidism and syndrome of inappropriate ADH secretion (SIADH).
This is associated with water intoxication, hyponatraemia and concentrated urine (osmolality ! 120–150 mmo/L). Hyponatraemia may be asymptomatic but levels of serum sodium below 120 mmol/L are associated with altered levels of consciousness, coma, seizure and a significant risk of death.
Causes of SIADH include malignancy, drugs.
Treatment of water excess and SIADH will usually
involve restriction of water intake to 600–800 ml/day. Hypertonic saline administration is rarely indicated.