Missed Periods: Labs, Diagnosis, and Next Steps

What Counts as a Missed Period

A single late cycle does not equal amenorrhea. The American College of Obstetricians and Gynecologists (ACOG) defines secondary amenorrhea as the absence of menstruation for three or more consecutive months in a person who has previously had regular cycles, or six months in someone with irregular cycles [1]. Primary amenorrhea refers to no menarche by age 15 when breast development and other secondary sexual characteristics are present, or by age 13 when they are absent [2].

Normal Cycle Variability

Menstrual cycles range from 21 to 35 days in adults. Variation of up to 7 days between cycles is considered physiologically normal [1]. Adolescents within two years of menarche and perimenopausal individuals routinely experience wider swings. A cycle that arrives at day 38 instead of day 30 is late, not missed.

When the Clock Starts

The three-month threshold exists because shorter gaps often self-correct. Ovulation can be delayed by acute illness, travel across time zones, or a single month of intense psychological stress. If a second cycle follows on schedule, no workup is needed. But once three months pass without a bleed, the probability of spontaneous resolution without intervention drops considerably, and bone and metabolic consequences begin accumulating [3].

Why Periods Stop: The Major Causes

The hypothalamic-pituitary-ovarian (HPO) axis governs menstruation. A disruption at any level of this axis, or in the uterus itself, can stop periods. Sorting out where the problem lies is the entire point of the lab workup.

Pregnancy

Always rule it out first. Even with reported contraceptive use, urine or serum beta-hCG remains step one. Approximately 50% of pregnancies in the United States are unintended, according to data from the Guttmacher Institute [4].

Hypothalamic Amenorrhea

Functional hypothalamic amenorrhea (FHA) results from suppression of gonadotropin-releasing hormone (GnRH) pulsatility. The Endocrine Society's 2017 clinical practice guideline identifies three primary triggers: energy deficit (from caloric restriction, eating disorders, or excessive exercise), psychological stress, and combinations of the three [5]. FHA affects an estimated 3% to 5% of adult women and up to 17% of those presenting with secondary amenorrhea [5].

The guideline states: "Functional hypothalamic amenorrhea is a diagnosis of exclusion that should be made only after organic causes have been ruled out" [5]. Lab findings typically show low or low-normal FSH and LH with low estradiol.

PCOS

Polycystic ovary syndrome is the single most common endocrine disorder in reproductive-age women, affecting 8% to 13% depending on diagnostic criteria used [6]. Under the Rotterdam criteria, a diagnosis requires two of three features: oligo-ovulation or anovulation, clinical or biochemical hyperandrogenism, and polycystic ovarian morphology on ultrasound [6]. PCOS-related amenorrhea is driven by chronic anovulation rather than estrogen deficiency, a distinction that matters for both bone health and treatment selection.

Thyroid Disease

Both hypothyroidism and hyperthyroidism disrupt menstrual regularity. A 2015 cross-sectional study in the Journal of Clinical Endocrinology & Metabolism (N=3,800) found that women with subclinical hypothyroidism had a 1.8-fold increased odds of menstrual irregularity compared with euthyroid controls [7]. TSH is part of every amenorrhea workup for this reason.

Hyperprolactinemia

Elevated prolactin suppresses GnRH. Causes include prolactinomas (the most common pituitary tumor), certain medications (antipsychotics, metoclopramide), and hypothyroidism itself. Prolactin levels above 100 ng/mL strongly suggest a prolactinoma and warrant pituitary MRI [8].

Other Causes

Premature ovarian insufficiency (POI) affects roughly 1% of women under 40 and presents with elevated FSH (typically above 25 mIU/mL on two measurements taken at least one month apart) [9]. Asherman syndrome (intrauterine adhesions), congenital anatomic anomalies, and medications such as hormonal contraceptives, GnRH agonists, and high-dose progestins also cause amenorrhea.

The Lab Workup: What to Order and Why

A structured, stepwise approach prevents both under-testing and the "shotgun panel" that generates confusion without clarity. The ACOG Practice Bulletin on amenorrhea provides a clear algorithm [1].

First-Tier Labs

Order these for every patient presenting with missed periods:

• Beta-hCG (serum or urine): rules out pregnancy
• TSH: screens for thyroid dysfunction
• Prolactin: screens for hyperprolactinemia
• FSH: distinguishes ovarian failure (high FSH) from hypothalamic/pituitary suppression (low/normal FSH)
• Estradiol (E2): confirms estrogen status, useful for bone risk stratification

This five-test panel covers the vast majority of treatable causes. Dr. Robert Barbieri, former chair of obstetrics and gynecology at Brigham and Women's Hospital, has written: "A pregnancy test, TSH, prolactin, and FSH will identify the cause of secondary amenorrhea in approximately 85% of cases" [10].

Second-Tier Labs (Based on Clinical Suspicion)

If first-tier results are inconclusive or if clinical signs point toward specific diagnoses, add:

• LH: an LH-to-FSH ratio of 2:1 or greater supports (but does not confirm) PCOS
• Total and free testosterone, DHEA-S: assess hyperandrogenism when PCOS is suspected
• 17-hydroxyprogesterone: screens for nonclassical congenital adrenal hyperplasia, which mimics PCOS in up to 5% of hyperandrogenic women [11]
• Anti-Müllerian hormone (AMH): elevated in PCOS, low in diminished ovarian reserve
• Comprehensive metabolic panel and fasting insulin: assesses metabolic risk in PCOS and FHA patients
• Cortisol (morning or 24-hour urine free cortisol): if Cushing syndrome is a concern

Imaging

Pituitary MRI is indicated when prolactin exceeds 100 ng/mL or when central pathology is suspected [8]. Pelvic ultrasound evaluates ovarian morphology and endometrial thickness. For primary amenorrhea with absent uterus on imaging, a karyotype (46,XY vs. 46,XX) becomes necessary.

The Progestin Challenge Test

Historically, clinicians used medroxyprogesterone 10 mg for 10 days to see if withdrawal bleeding occurred. Bleeding suggested adequate estrogen and an intact outflow tract. No bleeding pointed toward low estrogen or anatomic obstruction. This test is falling out of favor because it is unreliable as a standalone diagnostic tool. A negative progestin challenge does not reliably distinguish FHA from other causes, and a positive one does not rule out PCOS [1]. Lab-based evaluation has largely replaced it.

Interpreting Results: Pattern Recognition

Raw lab values matter less than the pattern they form together. Recognizing these patterns quickly narrows the differential.

Low FSH + Low Estradiol

This pattern points to hypothalamic or pituitary suppression. The brain is not sending the signal. In a young woman with recent weight loss or high training volume, FHA is the most likely explanation. In an older patient or one with headaches and visual changes, pituitary pathology enters the differential.

High FSH + Low Estradiol

The brain is sending a strong signal, but the ovaries are not responding. Two FSH measurements above 25 mIU/mL taken at least four weeks apart, in a woman under 40, meet the diagnostic criteria for premature ovarian insufficiency [9]. POI is confirmed, not merely suspected, at this threshold. These patients need estrogen replacement for bone and cardiovascular protection.

Normal FSH + Normal or High Estradiol + Elevated Androgens

This is the classic PCOS pattern. The ovary produces estrogen (often from peripheral aromatization of androgens in adipose tissue), but ovulation fails. Endometrial protection becomes a priority because unopposed estrogen raises the risk of endometrial hyperplasia.

Elevated TSH

Treat the thyroid first. Menstrual cycles frequently normalize once euthyroid status is restored with levothyroxine. A 2012 study in Thyroid found that 76% of hypothyroid women with menstrual irregularity resumed normal cycles within 6 months of achieving a TSH in the reference range [12].

Elevated Prolactin

Prolactin between 25 and 100 ng/mL may reflect medication effects, stress, or a microprolactinoma. Levels above 200 ng/mL almost always indicate a macroprolactinoma. Dopamine agonists (cabergoline or bromocriptine) are first-line treatment and normalize prolactin in over 80% of patients with prolactinomas [8].

Treatment by Cause

There is no single treatment for missed periods. The intervention matches the diagnosis.

Hypothalamic Amenorrhea

The Endocrine Society guideline recommends addressing the root behavioral or metabolic trigger [5]. That means increasing caloric intake, reducing exercise volume, or managing chronic stress. For patients who cannot or will not modify behavior, or for whom bone density is already compromised, transdermal estradiol (100 mcg patch) plus cyclic micronized progesterone provides estrogen replacement without suppressing the HPO axis the way combined oral contraceptives do [5].

Bone loss is the most urgent concern. Women with FHA lose trabecular bone at roughly 2% to 3% per year [3]. A DEXA scan is recommended after 6 months of amenorrhea. Weight restoration is the single most effective intervention for recovery of spontaneous menses, with one prospective study (N=186) showing that 73% of women with anorexia nervosa-related amenorrhea resumed menstruation within 12 months of reaching 90% of ideal body weight [13].

PCOS

Lifestyle modification is first-line for PCOS-related amenorrhea when BMI is above 25. A weight reduction of just 5% to 10% restores ovulatory cycles in a significant proportion of affected women [6]. For cycle regulation, options include:

• Combined oral contraceptives: regulate cycles and reduce androgen levels
• Cyclic progestins: medroxyprogesterone 10 mg for 10 to 14 days every 1 to 3 months provides endometrial protection
• Metformin: may restore ovulation, though the American Association of Clinical Endocrinologists (AACE) positions it as second-line behind oral contraceptives for cycle regulation [14]
• Letrozole or clomiphene: first-line for ovulation induction when pregnancy is desired. The NICHD Reproductive Medicine Network trial (N=750) demonstrated that letrozole produced higher live birth rates than clomiphene in women with PCOS (27.5% vs. 19.1%, P=0.007) [15]

Thyroid-Related Amenorrhea

Levothyroxine replacement at a starting dose of 1.6 mcg/kg/day for overt hypothyroidism. Subclinical cases with TSH between 4.5 and 10 mIU/L may be treated if menstrual irregularity is the chief complaint, though data supporting treatment at this threshold are less strong [7].

Hyperprolactinemia

Cabergoline 0.25 to 0.5 mg twice weekly is preferred over bromocriptine for tolerability and efficacy. Surgical resection is reserved for macroadenomas unresponsive to dopamine agonists or causing mass effect [8].

Premature Ovarian Insufficiency

Hormone replacement therapy (HRT) with estradiol and progesterone is standard until the average age of natural menopause (approximately age 51). The goal is skeletal protection, cardiovascular risk reduction, and symptom management. Spontaneous conception remains possible in 5% to 10% of POI patients, so contraception counseling is still relevant [9].

Bone Health and Long-Term Risks

Estrogen deficiency from any cause of amenorrhea accelerates bone loss. This is not a distant, theoretical risk.

How Fast Bone Loss Occurs

Peak bone mass is typically achieved by age 25 to 30. Amenorrheic athletes lose 2% to 6% of lumbar spine bone mineral density per year, comparable to the rate seen in early menopause [3]. A study published in the Journal of Bone and Mineral Research (N=214) found that collegiate athletes with amenorrhea had significantly lower lumbar spine Z-scores than eumenorrheic teammates (mean difference: -0.4 SD, P<0.001) [16].

When to Order a DEXA Scan

ACOG and the Endocrine Society both recommend baseline DEXA after 6 months of amenorrhea in reproductive-age women, with follow-up every 1 to 2 years if the underlying cause persists [1][5]. Do not wait for a stress fracture to prompt imaging.

Calcium and Vitamin D

Supplementation alone does not reverse amenorrhea-related bone loss, but deficiency worsens it. Aim for 1,000 to 1,200 mg calcium daily (dietary plus supplemental) and maintain serum 25-hydroxyvitamin D above 30 ng/mL [3].

When to Refer to a Specialist

Most cases of secondary amenorrhea can be worked up and managed in primary care or by a gynecologist. Referral to reproductive endocrinology is warranted when:

• FSH is persistently elevated in a woman under 40 (suspected POI)
• Prolactin is above 100 ng/mL or pituitary imaging is abnormal
• Primary amenorrhea with abnormal karyotype or absent Müllerian structures
• Fertility is desired and first-line ovulation induction has failed
• The cause remains unclear after a complete workup

For patients with eating disorders or exercise-related FHA, co-management with a dietitian and mental health provider is not optional. The Endocrine Society guideline emphasizes that pharmacologic treatment alone, without behavioral change, produces inferior outcomes in FHA [5].

Bone density at the 12-month mark after amenorrhea onset predicts long-term fracture risk more reliably than any single lab value [16].