July 23, 2017
Oral GTT for the Diagnosis of Growth Hormone Excess Minimize

Oral GTT for the diagnosis of growth hormone excess

Indication

Clinical suspicion of acromegaly or gigantism. Baseline GH values cannot be used to exclude acromegaly since elevated GH may be occur with stress and low values < 5 mIU/L are seen in up to 8% acromegalic patients who are subsequently identified by the failure of GH to suppress during GTT.

Contraindication

This test is unnecessary in diabetic patients who should already have a suppressed GH in the presence of hyperglycaemia.

Principle:

GH secretion is part of the counter-regulatory defence against hypoglycaemia and physiological GH secretion is inhibited by hyperglycaemia. In acromegaly, or gigantism, GH secretion is autonomous and does not suppress and may paradoxically rise with 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 water.

Requirements

Adults: 75 g anhydrous glucose in cold water. The solution should be chilled to improve palatability. An alternative is this Polycal (113 mL) which is still more palatable and should be followed by 150mL water (total volume should be 250-300mL)

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

Indwelling cannula gauge 19.
5 plain and 5 fluoride oxalate tubes

Procedure

 time 0 min   Insert an indwelling cannula and take blood samples for GH, IGF-1 and glucose (1-2 mL in plain & 1-2 mL in fluoride oxalate tubes).
   The glucose solution / Polycal should be drunk within 5 minutes.
 time 30 min  Take further blood samples for GH and glucose
 time 60 min  Take further blood samples for GH and glucose
 time 90 min  Take further blood samples for GH and glucose
 time 120 min  Take further blood samples for GH and glucose


Interpretation

Normal subjects will 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 values during 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).

Specific assay dependent cut-offs need to be used for interpretation. It should be noted that the most recent study reporting these cut-offs uses assays that are either not compliant with recommendations to use the International Reference Preparation 98/574 or are now unavailable (Arafat et al 2008).

Reference

  • Arafat AA, Mohlig M, Weickert MO, Perschel FH, Purschwitz J, Spranger J, Strasburger CJ, Schofl C, Pfeiffer FH. Growth hormone response during oral; glucose tolerance test: the impact of assay method on the estimation of reference values in patients with 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 with ‘normal’ serum growth hormone levels. Horm Metab Res 1992;24:392-396.
  • Freda PU. monitoring of acromegaly: what should be performed when GH and IGF-1 levels are discrepant? Clin Endocrinol 2009;71:166-170.

JHB 22 Sept 2011

  

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