Ovarian hyperstimulation syndrome (OHSS) is rather frequent (1-5%) in women submitted to superovulation with gonadotropins for in vitro fertilisation (IVF), whereas it is very rare in case of spontaneous ovulation. Spontaneous OHSS (sOHSS) was previously described to be associated to hydatiform mole, multiple conception, hypothyroidism in pregnancy. It may also depend on activating mutations of the FSH receptor (FSHR) gene that cause ovarian hyper-responsiveness to circulating FSH or even cross-responsiveness of FSHR to hormones having a structure similar to FSH, such as hCG or TSH. We report, herein, a case of sOHSS in a woman who conceived spontaneously. We checked the presence of all possible factors that could explain the onset of the syndrome, and we evidenced hypothyroidism and abnormally elevated hCG levels in the second trimester of pregnancy. The thorough molecular biology study of FSHR gene did not detect exonic mutations, but revealed the presence of intronic mutations whose role in the onset of sOHSS is still uncertain.