Missed Periods: What Could Be Causing It

Primary vs. Secondary Amenorrhea: Why the Distinction Matters

The clinical approach to missed periods depends on whether menses never started or stopped after a period of normal cycling. Primary amenorrhea refers to the absence of menstruation by age 15 in the presence of secondary sexual characteristics (or by age 13 without them). Secondary amenorrhea, the far more common presentation, is the cessation of previously regular periods for three or more consecutive months [1].

This distinction shapes the diagnostic workup. Primary amenorrhea often involves chromosomal or anatomical causes such as Turner syndrome (45,X karyotype) or Müllerian agenesis, conditions that require genetics referral and imaging [2]. Secondary amenorrhea, by contrast, usually stems from acquired hormonal disruptions. The American College of Obstetricians and Gynecologists (ACOG Practice Bulletin No. 128) recommends evaluating secondary amenorrhea after 3 months of missed menses in women with previously regular cycles, or after 6 months in those with a history of oligomenorrhea.

For the remainder of this article, the focus is secondary amenorrhea, since that matches the clinical scenario most people describe when they say they are "missing periods." The causes are identifiable, the labs are straightforward, and the treatments work.

The Most Common Causes of Missed Periods

Four conditions account for the vast majority of secondary amenorrhea cases: PCOS, hypothalamic amenorrhea, thyroid disease, and hyperprolactinemia. Pregnancy remains the single most frequent explanation and should always be excluded first with a serum or urine beta-hCG test before any other workup begins [3].

After pregnancy is ruled out, the differential breaks down roughly as follows. A retrospective analysis of 262 women evaluated for secondary amenorrhea found that hypothalamic amenorrhea accounted for 35%, PCOS for 26%, hyperprolactinemia for 13%, and ovarian failure for 10% of cases [4]. These numbers shift depending on the population studied. In adolescents, the hypothalamic-pituitary-ovarian axis may simply be maturing, while in women over 40, perimenopause or premature ovarian insufficiency becomes more likely.

Less common but clinically significant causes include Asherman syndrome (intrauterine adhesions, often post-procedural), Cushing syndrome, congenital adrenal hyperplasia, and medications such as depot medroxyprogesterone acetate, antipsychotics, and certain antiepileptics [5]. Drug-induced amenorrhea is underrecognized. Antipsychotics that block dopamine D2 receptors raise prolactin levels and can suppress ovulation within weeks of initiation.

Hypothalamic Amenorrhea: When the Brain Turns Off the Cycle

Hypothalamic amenorrhea (HA) occurs when the hypothalamus reduces or stops pulsatile secretion of gonadotropin-releasing hormone (GnRH). Without those pulses, the pituitary does not release adequate LH and FSH, and the ovaries go quiet. The result is low estradiol, absent ovulation, and no period.

Three overlapping triggers drive HA: caloric deficit, excessive exercise, and psychological stress. Many patients have more than one trigger simultaneously. The Endocrine Society Clinical Practice Guideline on functional hypothalamic amenorrhea (2017) defines it as a diagnosis of exclusion after organic pituitary and ovarian pathology have been ruled out [6]. The guideline states: "Functional HA is a form of chronic anovulation not due to identifiable organic causes but rather to a variety of stressors, including weight loss, exercise, and psychogenic stress."

Laboratory findings in HA typically show FSH in the low-normal range (often <5 mIU/mL), LH that is low or inappropriately normal, and estradiol below 50 pg/mL. These values distinguish HA from PCOS, where LH is often elevated relative to FSH, and from premature ovarian insufficiency, where FSH is high (typically >25 mIU/mL) [6].

The bone consequences of prolonged HA deserve attention. Women with HA for more than 6 months show significantly reduced lumbar spine bone mineral density compared to eumenorrheic controls. A study in the Journal of Clinical Endocrinology & Metabolism found that women with HA had 2-4% lower spine BMD per year of amenorrhea, a rate of loss comparable to early menopause [7]. This makes treatment more than a matter of cycle restoration. It is a bone-protection strategy.

Treatment for HA centers on addressing the root cause. Increasing caloric intake, reducing exercise volume, and managing stress can restore menses in many cases without pharmacologic intervention. The Endocrine Society recommends cognitive behavioral therapy (CBT) as a first-line psychological intervention. For patients who cannot or will not modify behavior, transdermal estradiol with cyclic progesterone protects bone while the underlying stressor is addressed [6].

PCOS: The Most Common Endocrine Cause

Polycystic ovary syndrome is the leading endocrine disorder in reproductive-age women, affecting 8-13% of this population depending on diagnostic criteria used [8]. Among women presenting specifically with amenorrhea or oligomenorrhea, PCOS is the cause in roughly one-quarter to one-third of cases.

Diagnosis follows the Rotterdam criteria (2003, reaffirmed by international guidelines in 2023): two of three features must be present. These are oligo- or anovulation, clinical or biochemical hyperandrogenism, and polycystic ovarian morphology on ultrasound (defined as 20 or more follicles per ovary using modern transducers or ovarian volume >10 mL) [9]. The 2023 international evidence-based guideline, endorsed by over 40 organizations, explicitly states that ultrasound alone is neither sufficient nor required for diagnosis if the other two criteria are met.

Dr. Helena Teede, chair of the 2023 PCOS guideline committee, noted: "PCOS remains underdiagnosed and poorly managed globally, with up to 70% of affected women undiagnosed in primary care." This gap means many women with missed periods attributed to "stress" actually have unrecognized PCOS [9].

The missed periods in PCOS result from chronic anovulation driven by insulin resistance and androgen excess. Elevated insulin stimulates ovarian androgen production, which disrupts follicular development and prevents regular ovulation. Treatment depends on the patient's goals. For cycle regulation without pregnancy intent, combined oral contraceptives remain first-line. Metformin (typically 1,500-2 to 000 mg daily) can improve ovulation rates, particularly in women with a BMI over 25 kg/m². For fertility, letrozole 2.5-7.5 mg daily (days 3-7) has replaced clomiphene as the preferred ovulation induction agent after the NEJM PPCOS II trial (N=750) demonstrated higher live birth rates with letrozole (27.5%) versus clomiphene (19.1%, P=0.007) [10].

Thyroid Disorders and Hyperprolactinemia

Both hypothyroidism and hyperthyroidism disrupt menstrual cyclicity. Hypothyroidism is the more common culprit. Elevated TSH stimulates thyrotropin-releasing hormone (TRH), which in turn raises prolactin, creating a secondary hyperprolactinemia that suppresses GnRH pulsatility. A cross-sectional study of 171 hypothyroid women found that 25.7% had menstrual irregularities, compared to 7% of euthyroid controls [11].

The fix is straightforward. Levothyroxine replacement that normalizes TSH (target 0.5-2.5 mIU/L in reproductive-age women) typically restores regular menses within 2-3 months. No additional hormonal therapy is needed for the amenorrhea itself.

Primary hyperprolactinemia, not driven by thyroid disease, has its own differential. Prolactinomas (pituitary lactotroph adenomas) are the most common cause, accounting for roughly 40% of all pituitary tumors [12]. Medications, particularly antipsychotics and metoclopramide, are the second most frequent trigger. The diagnostic threshold is a serum prolactin level above 25 ng/mL, though levels above 200 ng/mL strongly suggest a macroprolactinoma [12].

Dopamine agonists are the standard treatment for prolactinomas. Cabergoline normalizes prolactin in approximately 85% of patients and shrinks tumor volume by 50% or more in roughly two-thirds of macroprolactinomas. It is preferred over bromocriptine for better efficacy and tolerability (once- or twice-weekly dosing versus daily) [13]. Menses typically resume within 2-3 months of prolactin normalization.

When to See a Doctor: Clinical Red Flags

A single missed period in the context of recent stress, travel, or illness does not require immediate medical evaluation. Three consecutive missed periods, or 90 days without a bleed, meets the clinical threshold for investigation.

Certain scenarios warrant earlier or more urgent evaluation. Missed periods with severe headaches or visual field changes could indicate a pituitary mass compressing the optic chiasm. Missed periods combined with hot flashes, night sweats, and vaginal dryness in a woman under 40 suggest premature ovarian insufficiency (POI), which affects approximately 1% of women under 40 and 0.1% under 30 [14]. POI requires prompt diagnosis because of its implications for bone health, cardiovascular risk, and fertility planning.

Signs that should prompt same-week evaluation include galactorrhea (nipple discharge without pregnancy or breastfeeding), rapid or unexplained weight gain with central obesity (Cushing syndrome), and virilization (deepening voice, male-pattern hair growth accelerating over months). These patterns point to conditions where delayed diagnosis carries real clinical cost.

The ACOG recommendation is clear: evaluate after 3 missed cycles. Do not wait longer in hopes the period "comes back on its own," especially if the patient desires pregnancy or has risk factors for bone loss.

The Standard Diagnostic Workup

The initial evaluation for secondary amenorrhea is efficient. It begins with a pregnancy test (urine or serum beta-hCG), followed by a focused hormonal panel: TSH, prolactin, FSH, and estradiol [1]. These four tests, combined with the pregnancy test, narrow the differential substantially.

Interpreting the results follows a decision-tree logic. Elevated TSH points to hypothyroidism. Elevated prolactin (after ruling out thyroid-driven elevation and medications) suggests a prolactinoma, and pituitary MRI is the next step. Elevated FSH (>25-40 mIU/mL on two draws at least one month apart) indicates ovarian insufficiency or failure [14]. Low or normal FSH with low estradiol points to hypothalamic amenorrhea or a pituitary lesion. Normal or mildly elevated LH with clinical or biochemical hyperandrogenism suggests PCOS.

Additional tests ordered based on clinical context include total and free testosterone, DHEA-S (to exclude adrenal sources of androgen excess), 17-hydroxyprogesterone (to screen for non-classic congenital adrenal hyperplasia, which affects 1 in 200 to 1 in 1,000 individuals depending on ethnicity) [15], and a progestin challenge test. In the progestin challenge, administration of medroxyprogesterone acetate 10 mg for 10 days assesses whether the uterus can produce withdrawal bleeding, which indicates adequate estrogen priming and a patent outflow tract.

Pelvic ultrasound is recommended when PCOS is suspected (to assess ovarian morphology) or when outflow obstruction is a concern. Pituitary MRI is indicated for confirmed hyperprolactinemia or when pituitary pathology is clinically suspected.

Treatment: Matching Therapy to the Cause

There is no single treatment for missed periods because missed periods are a symptom, not a diagnosis. Therapy targets the underlying condition.

For hypothalamic amenorrhea, the evidence favors behavioral modification first: increasing caloric intake by 200-400 kcal/day above expenditure, reducing high-intensity exercise by 10-20%, and addressing psychological stressors through CBT [6]. A randomized trial showed that women who increased caloric intake recovered menses at a significantly higher rate than controls. For those who cannot modify behavior or who have bone density concerns, transdermal estradiol 100 mcg/day with cyclic micronized progesterone 200 mg for 12 days per month provides endometrial and skeletal protection without suppressing the HPO axis the way combined oral contraceptives do.

For PCOS, combined oral contraceptives (containing 20-35 mcg ethinyl estradiol) regulate the cycle and reduce androgen levels. Spironolactone 50-200 mg daily can be added for persistent hirsutism or acne. If pregnancy is desired, letrozole is first-line for ovulation induction [10].

For thyroid disease, levothyroxine replacement (starting dose typically 1.6 mcg/kg/day, adjusted every 6-8 weeks to normalize TSH) restores cycles without additional intervention [11].

For hyperprolactinemia, cabergoline 0.25-1.0 mg twice weekly titrated to normalize prolactin is the standard of care, with monitoring via serial prolactin levels and MRI if a tumor was identified [13].

For premature ovarian insufficiency, hormone therapy with estradiol and cyclic progesterone is recommended until the natural age of menopause (approximately age 51) to protect bone and cardiovascular health. The 2015 ESHRE guideline on POI emphasizes that this is replacement, not supplementation, and the risk-benefit profile differs substantially from postmenopausal hormone therapy [14].

Patients with Asherman syndrome require hysteroscopic adhesiolysis followed by estrogen therapy to promote endometrial regrowth. Success rates for restoring menses after hysteroscopic treatment range from 75-95% depending on adhesion severity [16].