Summary A 50-year-old woman developed pronounced IgA-deficiency and hypothyroidism after 131I treatment for Graves’ disease. The deficiency state was associated with a severe sinobronchial syndrome. Treatment with L-thyroxine resulted in a normal IgA concentration and a dramatic clinical improvement. Of the various possible underlying mechanisms, impaired synthesis of IgA light and heavy light chains seemed most probable. Impaired production of J-chain was excluded.