Chapter 7 The hypothalamus and the pituitary gland
Anterior pituitary hormones
Measurement of anterior pituitary hormones
In males, a normal plasma testosterone concentration indicates normal LH secretion. In hypopituitarism, plasma testosterone concentration is low and LH and FSH concentrations are normal or low. In premenopausal females, amenorrhoea with a low plasma oestradiol concentration and normal or low gonadotrophins suggests hypothalamic or pituitary dysfunction. A clomifene test (see p. 170) may help to distinguish between these. A normal ovulatory plasma progesterone concentration (see p. 176) indicates the integrity of the hypothalamo-pituitary–ovarian axis without the need for further testing; a history of regular, normal menstrual cycles also effectively excludes gonadotrophin deficiency. In normal postmenopausal women, plasma gonadotrophin concentrations are grossly elevated; in hypopituitarism, they are normal or low.
The integrity of the hypothalamo-pituitary–adrenal axis can also be tested using the IHT. A rise in plasma cortisol concentration to at least 550 nmol/L after adequate hypoglycaemia indicates a normal axis. It has been shown that if the basal (09:00 h) plasma cortisol concentration is <100 nmol/L, the cortisol response to hypoglycaemia is never normal, whereas it invariably is normal if the basal concentration is >400 nmol/L. A formal IHT may therefore not be necessary in patients whose basal plasma cortisol concentrations are outside the range 100–400 nmol/L. The protocol for the IHT is given in Figure 7.7. The short ACTH stimulation test (tetracosactide or Synacthen test, see p. 141), used primarily in the investigation of adrenal failure, has also been advocated as a test for ACTH deficiency. This may seem illogical, but the rationale is that ACTH deficiency causes adrenal atrophy and thus decreases adrenal responsiveness to ACTH. A good correlation between the results of the IHT and short ACTH stimulation tests has been demonstrated: a plasma cortisol concentration >550 nmol/L 30 min after the administration of synthetic ACTH (250 µg, i.v.) excludes ACTH deficiency. Experience with the low dose (1 µg) tetracosactide test in this context is presently limited, but it may be less sensitive in identifying partial failure of ACTH secretion.
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