Thyroid hyperfunction during growth hormone (GH) therapy has not been reported previously. We report a case of a 33-year-old female who developed autoimmune thyrotoxicosis during treatment with GH and with normalization of the thyroid function after discontinuation of GH therapy. After rechallenge with GH she again developed thyrotoxicosis, this time with presence of thyroid autoantibodies and without normalization of the thyroid function after stopping the GH treatment. Several possibilities for a causal relationship between GH therapy and development of autoimmune thyrotoxicosis are proposed.