There is a small number of studies that have reported abnormalities in endocrine function after a long-term gonadotropin-releasing hormone agonist (GnRHa) treatment in girls. This treatment is considered as safe and effective by most authors. We report our second case of unusual outcome of long-term GnRHa therapy in a girl with central precocious puberty (CPP) of idiopathic or familial etiology. She has received monthly depot of injections of triptorelin for a time period of 4 years. We have examined thyroid function by measuring serum levels of thyrotropin (TSH), thyroxine (T4), thyroid antibodies and ultrasound of thyroid gland. At the age of 11 years she developed a mild goiter and presented with autoimmune thyroiditis, having elevated thyroid antibodies and ultrasound of thyroid gland typical for Hashimoto thyroiditis. Having in mind these two cases, we suggest a closer monitoring of thyroid function in girls with CPP, before and during therapy with GnRH agonist.