Hyperprolactinemia occurs in approximately 25% of adult women with secondary amenorrhea but is a much less common cause of oligomenorrhea in adolescents Nevertheless, the possibility of hyperprolactinemia must be considered in all adolescents with oligomenorrhea because only 40% to 50% of hyperprolactinemic patients have spontaneous or expressible galactorrhea Hyperprolactinemia can be a side effect of several commonly used medications (see Table 51.3 ) A pituitary adenoma is a rare, but important cause of hyperprolactinemia In one retrospective review of teenagers with pituitary adenomas, over 50% of the females presented with either oligomenorrhea or secondary amenorrhea Other central nervous system tumors should also be considered as a potential cause for hyperprolactinemia The occasional patient with galactorrhea but with a normal prolactin level should be reevaluated periodically in an effort to identify a treatable cause of the problem Ovarian Disorders Ovarian failure in adolescents may be caused by primary or acquired etiologies Primary ovarian failure most commonly is due to gonadal dysgenesis from genetic causes, most commonly Turner syndrome Secondary causes of premature ovarian failure include sequelae of chemotherapy, pelvic irradiation, or autoimmune disease Ovarian tumors or other hormone-secreting tumors may result in ovarian failure Endometrial destruction that results from overly vigorous curettage or pelvic tuberculosis is an exceedingly rare cause of oligomenorrhea Thyroid Disorders Hypothyroidism and hyperthyroidism can both produce menstrual irregularities The provider who sees a female patient presenting with either infrequent bleeding or excessive bleeding should have hypothyroidism and hyperthyroidism on the differential diagnosis It can be beneficial to send thyroid studies (TSH) from the emergency department Miscellaneous Among adolescents who not have overt signs of PCOS, hyperprolactinemia, or malnutrition, suppression of the hypothalamic–pituitary axis is the most common cause of oligomenorrhea that occurs more or persists for at least years after menarche Although oligomenorrhea in otherwise normal-appearing adolescents has historically been ascribed to psychosocial stressors (family disruption, moving, depression), many patients with apparently psychogenic menstrual irregularity prove on careful evaluation to have disordered eating patterns or