Immunoglobulin G

When high

High IgG:

• Serum protein electrophoresis with immunofixation -- the essential next test, distinguishing a polyclonal rise (infection, inflammation, liver disease) from a monoclonal band (MGUS, myeloma)
• Liver screen -- chronic liver disease, particularly autoimmune hepatitis, characteristically raises IgG; relevant given oral AAS hepatotoxicity
• Chronic infection screen -- HIV, hepatitis B and C, tuberculosis where the history fits
• Autoimmune screen -- ANA, rheumatoid factor if symptoms suggest connective tissue disease
• Reduce alcohol -- alcohol intake independently raises immunoglobulin levels

A monoclonal band requires haematology referral. A modest polyclonal rise with a clear inflammatory or infective cause is usually managed by treating that cause.

When low

Low IgG (hypogammaglobulinaemia):

• Confirm on a repeat sample with IgA and IgM -- an isolated low IgG differs from a pan-low pattern; the combination points to the underlying cause
• Review AAS dose and duration -- supraphysiological dosing suppresses antibody production; reducing dose or coming off is the primary modifiable factor
• Exclude protein loss -- nephrotic syndrome (check urine protein) and protein-losing enteropathy both drain IgG
• Review medications -- corticosteroids, rituximab, and some anticonvulsants cause secondary antibody deficiency

Supportive measures:

• Adequate protein intake -- 1.6-2.2g/kg/day; antibody synthesis is protein-dependent
• Zinc -- 15-30mg/day if deficient; zinc deficiency impairs both cellular and humoral immunity
• Vitamin D -- correct to 75-125 nmol/L; deficiency is associated with impaired immune function
• Sleep and training load -- chronic sleep restriction and unrecovered training volume both suppress antibody responses

Refer to an immunologist if IgG is persistently below 6 g/L, or at any level with recurrent bacterial infections, bronchiectasis, or poor vaccine responses. Immunoglobulin replacement therapy is a specialist decision.