What is the likely cause of secondary amenorrhea (absence of menstruation) in a 22-year-old female with a history of irregular menstrual periods (oligomenorrhea), normal thyroid-stimulating hormone (TSH) level, mildly elevated prolactin level, and negative human papillomavirus (HPV) test, and does she require further evaluation or referral to a gynecologist (GYN)?

Polycystic Ovary Syndrome (PCOS): Given the patient's history of irregular periods and the presence of a mildly elevated TSH, which can be associated with metabolic syndrome often seen in PCOS, this diagnosis is plausible. The normal prolactin level and negative pregnancy test also support this, as PCOS is a common cause of oligomenorrhea or amenorrhea in women of reproductive age.

Hypothyroidism: Although the TSH is only mildly elevated at 3.9, which is often considered within the normal range by many labs, some patients may exhibit symptoms of hypothyroidism with TSH levels at the higher end of the normal range. This could potentially contribute to menstrual irregularities.
Stress or Weight-Related Amenorrhea: Stress and significant weight changes can lead to hypothalamic amenorrhea, characterized by the cessation of menstrual cycles. This could be a contributing factor, especially if the patient has experienced recent life changes or alterations in her weight or exercise regimen.
Hyperprolactinemia (Mild): Although the prolactin level is within the normal range, some sources may consider 13.40 on the higher end, depending on the assay used. Mild hyperprolactinemia can sometimes cause menstrual irregularities.

Pituitary Tumor: Although less likely given the normal prolactin level, a pituitary tumor (especially a microadenoma) could cause menstrual irregularities. It's crucial not to miss this diagnosis due to its potential impact on health.
Premature Ovarian Failure (POF): This condition, now more commonly referred to as Premature Ovarian Insufficiency (POI), is characterized by the loss of ovarian function before the age of 40. It's a critical diagnosis not to miss due to its implications for fertility and long-term health.
Thyroid Dysfunction (More Severe): While the TSH is mildly elevated, ensuring that thyroid function is fully evaluated is important, as untreated thyroid disease can have significant health implications.

Congenital Adrenal Hyperplasia (CAH): A group of inherited disorders that affect the adrenal glands, leading to hormone imbalances. This could potentially cause menstrual irregularities but is less common.
Androgen Insensitivity Syndrome: A rare condition where an individual is genetically male (XY chromosomes) but has a partial or complete inability to respond to androgens, leading to the development of female external genitalia. This would be an extremely rare cause of primary amenorrhea but is worth considering in the differential diagnosis of menstrual irregularities in young women.

Given the patient's presentation, further workup may include:

A detailed history to assess for symptoms of thyroid disease, PCOS, or other endocrine disorders.
Physical examination to look for signs of androgen excess (in the case of PCOS), thyroid disease, or other abnormalities.
Laboratory tests:
- Free T4 to further evaluate thyroid function.
- Androgen levels (e.g., testosterone) if PCOS is suspected.
- Consider a pelvic ultrasound if there's suspicion of anatomical abnormalities or to evaluate ovarian morphology in PCOS.
Referral to a gynecologist or an endocrinologist may be beneficial for further evaluation and management, especially if the diagnosis is unclear or if specialized care is needed.

Referral to a gynecologist is warranted if:

The diagnosis is unclear after initial evaluation.
There are suspicions of anatomical abnormalities or other gynecologic issues that require specialized evaluation.
The patient desires fertility counseling or has concerns about fertility.
Management of the condition requires hormonal treatments or other interventions that are beyond the scope of primary care.