The association between primary hypothyroidism and precocious puberty secondary to ovarian hyperstimulation has been recognized for over a century. Here, we report the case of a 9-year-old girl with severe primary hypothyroidism, who presented with premature menarche, enlarged pituitary gland, enlarged ovaries with multiple cysts, and elevated prolactin and α-feto protein levels. Pituitary and ovarian radiology findings, and α-feto protein levels normalized a few weeks after hypothyroidism treatment was started. Reviewing the literature we found several reports of increased levels of tumor markers in girls with this association. Thyroid function tests should be always part of the evaluation of patients with precocious puberty especially if the bone age is delayed. Tumor markers and liver function tests may be abnormal in patients with severe hypothyroidism and improve soon after thyroid hormone replacement is started.