OBJECTIVE: Atypical forms of hyperthyroidism represent a diagnostic challenge for clinicians. Struma ovarii is an ovarian teratoma and constitutes a rare cause of ectopic thyroidal hormonal production. We describe a case of struma ovarii that combined two different sources of hyperthyroidism in the same patient and report genetic studies in order to contribute a better understanding of the autonomy and tumorigenesis of the struma ovarii. CASE REPORT: A 73-year-old nulliparous woman presented a thyroid toxic adenoma that was successfully treated with 10 mCi radioiodine. Unexpectedly, a new onset of hyperthyroidism prompted us to look for a second etiology. A whole-body scan with (123)I detected a pelvic hyperfixation suggesting struma ovarii, and a thyroid differentiated left ovarian teratoma 3 cm in size was surgically removed. We screened for mutations of thyroid-stimulating hormone receptor and Gs-alpha protein genes, as these mutations are common in thyroid adenomas. We did not identify any mutations. Androgen receptor study demonstrated a monoclonal status. Comparative genomic hybridization did not reveal any chromosomal abnormality. However, loss of heterozygosity analysis showed several structural abnormalities, compared with the majority of benign ovarian teratomas, which show a normal karyotype. CONCLUSIONS: This is the first well-documented report of thyrotoxic struma ovarii revealed after treatment of a single thyroid toxic adenoma. We have shown in this case that struma ovarii originates from a single germ cell, and, albeit benign, this tumor presents several chromosomal abnormalities. Struma ovarii-induced hyperthyroidism is likely to be mediated by mechanisms different from those of the classical thyroid toxic adenoma.