Growth hormone (GH) occurs in a number of isoforms. The pattern of isoforms present may transiently change in an athlete who abuses growth hormone. The authors are from the University of Southampton in England.
Isoforms of GH:
(1) 22 kiloDalton
(2) 20 kiloDalton
(3) acidic GH
(4) fragmented GH
(5) modified GH
Recombinant human growth hormone is primarily the 22 kiloDalton molecule. Injection of recombinant human GH suppresses endogenous release of GH isoforms. This increases the relative proportion of the 22 kiloDalton molecule in the serum.
Testing requires 2 assays:
(1) one specific for the 22 kiloDalton molecule
(2) one able to detect all of the isoforms (“permissive”)
ratio of 22 kiloDalton to all forms =
= (concentration of the 22 kiloDalton assay) / (concentration by the permissive assay)
Interpretation:
• Normally the ratio is < 0.8.
• During therapy with recombinant human GH the ratio is >= 1.0.
• The analytical sensitivity of the permissive assay for the 22 kD isoform must be lower than that of the specific assay, otherwise the ratio would never exceed 1.0
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