Combined Pituitary Function Tests (CPT)
Assessment of all components of anterior pituitary function used particularly in pituitary tumours or following tumour treatment.
Ischaemic heart disease.
Untreated hypothyroidism (impairs the GH and cortisol response).
Sweating, palpitations, loss of consciousness and rarely convulsions with hypoglycaemia.
Patients should be warned that with the TRH injection they may experience transient symptoms of: a metallic taste in the mouth, flushing and nausea.
The patient should fast overnight and be recumbent during test.
ECG must be normal and the patient's weight known.
In peri-pubertal children (bone age >10 years) priming is needed
M: 100 mg testosterone i.m. 3 days before testing
F: 100 mcg ethinyloestradiol p.o. each for three days before the test.
Calculate Actrapid Insulin dose:
Normal pituitary function 0.15 U/kg
Hypopituitary 0.10 U/kg
Acromegaly, diabetes, Cushing's 0.2-0.3 U/kg
TRH (Roche) 200 micrograms as slow i.v. injection.
LH/FSH releasing hormone (GnRH) - 100 mcg as i.v. bolus.
50mls 50% dextrose available for immediate administration.
6 fluoride tubes (grey top Vacutainers).
7 clotted tubes (red top Vacutainers) for samples.
500 ml bag 0.9% saline to flush cannula.
3 way tap to assist the taking of samples.
- 1. Site indwelling cannula.
- Take baseline blood samples for testosterone/oestradiol, prolactin, thyroxine, LH, FSH, TSH, GH, cortisol (14 ml clotted) and glucose (2 ml fluoride).
- Then at T = 0 inject insulin and GnRH i.v. as boluses followed by the TRH over 2 minutes.
- Take samples for LH, FSH, TSH, prolactin, GH, cortisol (7 ml clotted) and glucose (2 ml fluoride) at 30, 60 minutes and GH, cortisol, glucose at 90 and 120 minutes.
- Flush the cannula with saline between samples.
- At 30 minutes check blood glucose with Glucometer and repeat the insulin dose if not hypoglycaemic. Adequate hypoglycaemia (=2.2mmol/l) should be symptomatic. Record symptoms in the notes.
- Hypoglycaemia should be reversed by giving i.v. 50% dextrose, or i.m. glucagon (1 amp) and continue sampling. Take further samples for GH, cortisol and glucose at 90 and 120 minutes. There must be at least 2 specimens following adequate hypoglycaemia.
- At all times a doctor or nurse must be in attendance.
- If the patient has a hypoadrenal crisis with hypotension then they should be given i.v. 0.9% saline and hydrocortisone.
- Once test completed, give supervised meal.
- Patient should not drive for 2 hours after the test.
"SPLIT" COMBINED PITUITARY FUNCTION TEST
This test is no longer used, but the details are kept for historical reasons and for those sitting the part 1 MRCP (!) In patients with prolactinomas, and in some acromegalics with near-normal GH levels, it is useful to monitor the responses to TRH and GnRH alone. This test is only useful after treatment if it is known there was an abnormal test result prior to treatment.
The interpretation of the different components of the standard CPT is listed under the insulin tolerance test, the TRH test and the GnRH test.
In the "split" protocol it is possible to observe the isolated response of GH and Prolactin to TRH. In normals the prolactin will rise by 100% of its basal value while in patients with prolactinomas there is frequently a subnormal response. In normals there is a reduction in GH with TRH but there is a rise in 80% of acromegalics. It is only worth using the "split" protocol on a patient following treatment if they were tested by this protocol pre-treatment. The loss of the paradoxical rise to TRH in acromegaly is a good indicator of successful treatment.