What could be causing a late menstrual period in a female patient of reproductive age with negative pregnancy tests and potentially irregular menstrual cycles or underlying medical conditions such as Polycystic Ovary Syndrome (PCOS) or thyroid disorders?

Late Period with Negative Pregnancy Tests

A late menstrual period with negative pregnancy tests most commonly indicates ovulatory dysfunction, but requires systematic evaluation to exclude pregnancy (particularly early or ectopic), thyroid disorders, hyperprolactinemia, and polycystic ovary syndrome (PCOS). 1

Initial Critical Step: Confirm Pregnancy Status

• Repeat pregnancy testing is essential because false negatives occur in 1.6% of cases overall and 3.6% in patients with abdominal pain or vaginal bleeding, with ectopic pregnancies particularly at risk of being missed 2
• Modern urine pregnancy tests detect hCG 3-4 days after implantation, with 98% positive by the expected period, but a negative result one week after the missed period virtually guarantees non-pregnancy 3
• If high clinical suspicion persists (abdominal pain, vaginal bleeding, or risk factors for ectopic pregnancy), obtain a serum β-hCG regardless of negative urine test, as false negatives have been documented with serum levels ranging from 2 mIU/mL to well above assay detection 2

Core Laboratory Evaluation

Once pregnancy is excluded, proceed with hormonal assessment:

Essential First-Line Tests

• Thyroid function (TSH, free T4) to exclude thyroid dysfunction, which commonly presents with menstrual irregularity 1
• Prolactin level to rule out hyperprolactinemia, which suppresses GnRH pulsatility and causes anovulation mimicking PCOS; confirm elevation with 2-3 morning resting samples taken 20-60 minutes apart to exclude stress-related spurious elevation 1, 4
• FSH and LH measured between cycle days 3-6 (if cycle occurs), with three estimations 20 minutes apart for accuracy; an LH/FSH ratio >2 suggests PCOS 1, 5

Additional Tests Based on Clinical Presentation

• Mid-luteal progesterone (approximately day 21 of a 28-day cycle, or 7 days before expected menses) to confirm ovulation; levels <6 nmol/L indicate anovulation 1
• Testosterone and androstenedione if signs of hyperandrogenism are present (hirsutism, acne, male-pattern hair loss) 1, 5
• Fasting glucose and insulin with calculation of glucose/insulin ratio if PCOS or metabolic disorders suspected; a ratio >4 suggests insulin resistance 1, 4

Diagnostic Framework for Common Causes

Polycystic Ovary Syndrome (PCOS)

PCOS is the most common endocrine disorder causing irregular cycles, affecting approximately 6% of reproductive-age women 6:

• Clinical features: Oligomenorrhea (cycles >35 days), hyperandrogenism (hirsutism, acne), obesity with increased waist-hip ratio >0.9 5
• Laboratory findings: LH/FSH ratio >2, elevated free testosterone, low mid-luteal progesterone indicating anovulation 1, 5
• Imaging: Transvaginal ultrasound showing ≥20 follicles (2-9mm) in at least one ovary or ovarian volume ≥10mL 5
• Metabolic screening: Assess for insulin resistance, dyslipidemia, and cardiovascular risk factors even with apparently regular cycles 5

Hyperprolactinemia

• Inhibits gonadotropin secretion by suppressing kisspeptin neurons, directly blocking GnRH pulsatility and causing anovulation 4
• Rule out secondary causes: Measure TSH/free T4 for primary hypothyroidism, review medications (antipsychotics, metoclopramide), assess renal and hepatic function 4
• Treatment priority: Address primary hypothyroidism first, as this alone may normalize prolactin and restore menses 4

Thyroid Dysfunction

• Both hyperthyroidism and hypothyroidism impair ovarian function through direct effects and alterations in sex hormone binding protein levels 7
• Autoimmunity may be involved in the mechanism of infertility 7

Timing Considerations for Testing

• Perform all laboratory tests without hormonal contraception for accurate assessment 1
• For women with irregular cycles, still aim for days 3-6 testing when possible 1
• A single FSH measurement is not diagnostic as levels fluctuate significantly, particularly during perimenopause 1
• Normal ranges vary by laboratory; interpret based on specific reference ranges provided 1

When Imaging is Indicated

• Pelvic ultrasound (transvaginal preferred) between cycle days 3-9 if ovarian pathology suspected, looking for polycystic ovaries (>10 peripheral cysts 2-8mm diameter with thickened stroma) 1
• Pituitary MRI if clinical features or laboratory results suggest hypothalamic-pituitary abnormalities 1

Treatment Approach for Ovulatory Dysfunction

The PALM-COEIN classification system guides management of abnormal uterine bleeding 8:

• Medical treatments: Progestin-only contraception or combined hormonal contraception for abnormal uterine bleeding associated with ovulatory dysfunction 8
• Metformin for insulin resistance if fasting glucose/insulin ratio >4, particularly in PCOS 4
• Surgical options (endometrial ablation, hysterectomy) if medical treatment fails, is contraindicated, or not tolerated, or if concomitant significant intracavitary lesions exist 8

Critical Pitfalls to Avoid

• Accepting a single negative pregnancy test in patients with abdominal pain or vaginal bleeding without considering serum β-hCG, as false negatives are nearly 4 times more common in this population 2
• Relying on a single FSH measurement for diagnosis, particularly in perimenopausal women 1
• Measuring hormone levels at incorrect cycle times, which invalidates interpretation 1
• Failing to screen for thyroid disorders and hyperprolactinemia before attributing symptoms to PCOS 1, 4
• Not accounting for medication effects (particularly antipsychotics, depot medroxyprogesterone acetate) on menstrual patterns 8, 4

Specialist Referral Indications

• Persistent abnormal hormone levels or severe menstrual irregularities warrant endocrinology or gynecology referral 1
• Signs of premature ovarian insufficiency (amenorrhea with elevated FSH in women under 40) require specialist evaluation 1
• Infertility concerns necessitate reproductive endocrinology consultation 1