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February 25, 2015

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Oral GTT for the Diagnosis of Growth Hormone Excess

Oral GTT for the diagnosis of growth hormone excessIndication

Clinical suspicion of acromegaly or gigantism. Baseline GH values cannot be used to exclude acromegalysince elevated GH may be occur w ith stress and low values < 5 mIU/L are seen in up to 8% acromegalicpatients w ho are subsequently identif ied by the failure of GH to suppress during GTT.

Contraindication

This test is unnecessary in diabetic patients w ho should already have a suppressed GH in the presence ofhyperglycaemia.

Principle:

GH secretion is part of the counter-regulatory defence against hypoglycaemia and physiological GHsecretion is inhibited by hyperglycaemia. In acromegaly, or gigantism, GH secretion is autonomous and doesnot suppress and may paradoxically rise w ith hyperglycaemia.

Side effects

Some subjects feel nauseated and may have vaso-vagal symptoms during this test.

Preparation

Patients should be advised to fast for 10-16 hours prior to this test but may drink small volumes of w ater.

Requirements

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Adults: 75 g anhydrous glucose in cold w ater. The solution should be chilled to improve palatability. Analternative is this Polycal (113 mL) w hich is still more palatable and should be follow ed by 150mL w ater (totalvolume should be 250-300mL)

Children: the dose is w eight related 1.75g/kg body w eight: the maximum load is 75g.Polycal ® contains 0.66g anhydrous glucose per mL (or 1.51mL contains 1g anhydrous glucose).

Indw elling cannula gauge 19.5 plain and 5 f luoride oxalate tubes

Procedure

time 0min

Insert an indw elling cannula and take blood samples for GH, IGF-1 and glucose (1-2 mL in plain &1-2 mL in f luoride oxalate tubes).

The glucose solution / Polycal should be drunk w ithin 5 minutes.

time 30min Take further blood samples for GH and glucose

time 60min Take further blood samples for GH and glucose

time 90min Take further blood samples for GH and glucose

time 120min Take further blood samples for GH and glucose

Interpretation

Normal subjects w ill exhibit suppression of GH to undetectable values (< 0.3 µg/L) during the test.NB paradoxical rise in GH may occur during GTT during normal adolescence.

Sensitivity and Specificity

Fasting GH may be normal in 8% of acromegalic subjects but GH does NOT supress to undetectable valuesduring this test (Brockmeier et al 1992).

GH may fail to suppress due to chronic renal failure, liver failure, active hepatitis, anorexia nervosa,malnutrition, hyperthyroidism, diabetes and adolescence (Freda 2009).

Specif ic assay dependent cut-offs need to be used for interpretation. It should be noted that the most recentstudy reporting these cut-offs uses assays that are either not compliant w ith recommendations to use the

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International Reference Preparation 98/574 or are now unavailable (Arafat et al 2008).

Reference

Arafat AA, Mohlig M, Weickert MO, Perschel FH, Purschw itz J, Spranger J, Strasburger CJ, Schofl C,Pfeiffer FH. Grow th hormone response during oral; glucose tolerance test: the impact of assaymethod on the estimation of reference values in patients w ith acromegaly and in healthy controls,and the role of gender, age, and body mass index. J Clin Endocrinol Metab 2008;93:1254-1262.Brockmeier SJ, Buchfelder M, Adams EF, Schott W, Fahlbusch R. Acromegaly w ith ‘normal’ serumgrow th hormone levels. Horm Metab Res 1992;24:392-396.Freda PU. monitoring of acromegaly: w hat should be performed w hen GH and IGF-1 levels arediscrepant? Clin Endocrinol 2009;71:166-170.

JHB 22 Sept 2011

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