Gonadotrophin Releasing Hormone GnRH/LHRH Test

INDICATION

1. To further investigate possible gonadotrophin deficiency.
2. To confirm precocious puberty.

CONTRAINDICATIONS

None

PREPARATION

Overnight fast not necessary if done alone.
In women with a normal menstrual cycle the test should be performed in the follicular phase (day 3-7 of the cycle).
Larger dose or priming with LHRH if suspected of hypogonadism may be necessary.
(N.B. Do not prime with sex steroids if indication 2 above)
100 mcg LHRH (GnRH - Gonadorelin).
3 clotted tubes (red top Vacutainers - 7 ml)

SIDE EFFECTS

Nil

METHOD

1. Site indwelling cannula.
2. Take baseline bloods: LH, FSH and testosterone (M) or oestradiol (F).
3. Inject GnRH intravenously.
4. Flush cannula with saline.
5. Take samples for LH and FSH at t = 30 and 60 mins.

INTERPRETATION

• The normal peaks can occur at either 30 or 60 minutes. LH should exceed 10 U/l and FSH should exceed 2 U/l. An inadequate response may be an early indication of hypopituitarism.
• Gonadotrophin deficiency is diagnosed on the basal levels rather than the dynamic response. In males this is based on low testosterone in the absence of raised basal gonadotrophins and in females low oestradiol without elevated basal gonadotrophins and no response to clomiphene.
• Pre-pubertal children should have no response of LH or FSH to LHRH. If sex steroids are present (i.e. the patient is undergoing precocious puberty), the pituitary will be "primed" and will therefore respond to LHRH. It is important to stress that priming with steroids MUST NOT occur before this test.

SENSITIVITY AND SPECIFICITY

This test has a low sensitivity and specificity for hypogonadotrophic hypogonadism. The response may be normal or even exaggerated (especially in patients with hypothalamic disease). Basal levels are better discriminants. Serial investigations in patients with pituitary disease especially irradiation may give early indication of the development of hypopituitarism.

REFERENCE

Mortimer et al., BMJ, 3: 267-271