Hypothalamic Amenorrhea: Given the patient's secondary amenorrhea, normal LH and FSH levels, and a slightly elevated prolactin level, hypothalamic amenorrhea is a strong consideration. This condition is often associated with stress, weight loss, or excessive exercise, leading to a disruption in the hypothalamic-pituitary-gonadal axis.

Polycystic Ovary Syndrome (PCOS): Although the provided testosterone level is pending, the patient's LH level is at the lower end of the normal range, and the FSH level is slightly elevated, which could suggest PCOS, especially if the testosterone level comes back elevated. PCOS is a common cause of secondary amenorrhea.
Prolactinoma: The prolactin level is slightly elevated, which could indicate a prolactinoma, a type of pituitary tumor that secretes prolactin. This condition can cause secondary amenorrhea due to the inhibitory effect of prolactin on the hypothalamic-pituitary-gonadal axis.
Thyroid Dysfunction: Although not directly indicated by the provided labs, thyroid dysfunction can cause secondary amenorrhea. It would be essential to check thyroid function tests (TSH, free T4) to rule out hypothyroidism or hyperthyroidism.

Pregnancy: Despite the beta hCG level being <1 mIU/mL, which suggests that pregnancy is unlikely, it is crucial to confirm this with a urine pregnancy test, especially if there has been any chance of conception. Missing a pregnancy, especially an ectopic pregnancy, could have severe consequences.
Pituitary Adenoma (non-prolactinoma): Other types of pituitary adenomas, such as those secreting LH or FSH, or non-functioning adenomas, could also cause secondary amenorrhea by disrupting the normal function of the pituitary gland.

Congenital Adrenal Hyperplasia (CAH): This is a group of rare genetic disorders that affect the adrenal glands, leading to an imbalance in the production of sex hormones. Some forms of CAH can present with secondary amenorrhea, although this would typically be associated with other symptoms such as hirsutism or virilization.
Androgen Insensitivity Syndrome: A rare condition where an individual is genetically male (XY) but has a resistance to androgens, leading to the development of female secondary sexual characteristics but with undescended testes and no uterus, which would result in primary amenorrhea. However, in some cases, it might be considered in the differential for secondary amenorrhea if there's a history of previous menstrual cycles.
Gonadal Dysgenesis: Conditions like Turner syndrome (45,X) can cause ovarian dysgenesis leading to secondary amenorrhea due to the lack of functional ovarian tissue. However, these conditions typically present with primary amenorrhea.