The most appropriate management is to initiate estrogen and progestin therapy in this patient with primary amenorrhea. Primary amenorrhea is defined as the lack of menses by age 16 years accompanied by a normal body hair pattern and normal breast development. Pregnancy must be ruled out in all patients with primary amenorrhea. Approximately 50% of patients with primary amenorrhea have a chromosomal abnormality. Primary ovarian insufficiency due to Turner syndrome, a syndrome characterized by short stature and the loss of a portion or all of one X chromosome, is one of the most common causes of primary amenorrhea. The diagnosis of Turner syndrome can be made on the basis of a karyotype, and this should be the next diagnostic test for this patient. Such a patient may also have fragile X premutation; however, no cognitive impairment is typically seen in this patient population. Diagnosing Turner syndrome is critical because affected patients have a higher incidence of cardiovascular disease, metabolic syndrome, and thyroid dysfunction. Patients with Turner syndrome may have either primary or secondary amenorrhea and commonly have normal secondary sexual characteristics. The mechanism involved appears to be early follicular depletion, such that ovaries are devoid of follicles and oocytes. Serum evaluations in these patients will reveal low estradiol levels (typically below the detectable level in the assay) and markedly elevated gonadotropin levels. This constellation of findings is consistent with hypergonadotropic hypogonadism. Such patients should receive hormone replacement therapy with estrogen and cyclic progestin to prevent endometrial hyperplasia, osteoporosis, and other sequelae of hypoestrogenism.

A pituitary prolactinoma causes secondary amenorrhea through direct inhibition of gonadotropin-releasing hormone secretion by prolactin. Because this patient has elevated levels of gonadotropins, neither a pituitary MRI nor a prolactin measurement is necessary.

Both hypothyroidism and hyperthyroidism also cause secondary amenorrhea. Hypothyroidism results in increased levels of thyrotropin-releasing hormone through negative feedback, and this hormone, in turn, stimulates prolactin secretion and suppresses gonadotropin secretion. Hyperthyroidism can cause rapid weight loss, which is known to cause functional hypothalamic amenorrhea. Since this patient has elevated gonadotropin levels and no signs of hyperthyroidism, thyroid-stimulating hormone measurement is not needed.