What perimenopause actually is
Perimenopause is the transitional period leading up to menopause — defined as 12 consecutive months without a menstrual period. The Stages of Reproductive Aging Workshop (STRAW+10) criteria, the gold standard staging system, divides the menopausal transition into early and late stages based on menstrual cycle changes and hormone levels[1].
It typically begins in the early-to-mid 40s but can start as early as the late 30s. The average duration is 4-8 years, with some women experiencing symptoms for over a decade. During this time, ovarian function declines unevenly — not in a smooth downward line, but in erratic swings that produce the hallmark symptoms: irregular cycles, hot flashes, sleep disruption, mood changes, brain fog, and joint pain.
The challenge with blood work during perimenopause is that hormone levels can vary dramatically from month to month, and even week to week. A single snapshot can be misleading. Understanding which markers to track, when to test, and how to interpret fluctuating results is the difference between useful information and noise.
The core hormone panel
FSH (follicle-stimulating hormone)
FSH is the pituitary hormone that stimulates the ovaries to develop follicles and produce estrogen. As ovarian reserve declines, the pituitary compensates by producing more FSH — like pressing harder on the accelerator when the engine is losing power. An early follicular phase (day 2-5) FSH above 25 IU/L is suggestive of perimenopause; above 40 IU/L sustained over time indicates menopause[1].
The caveat: FSH fluctuates significantly during perimenopause. It can be 45 IU/L one month and 15 IU/L the next, because the ovaries still produce estrogen intermittently, which suppresses FSH via negative feedback. A single elevated FSH does not confirm menopause, and a single normal FSH does not rule out perimenopause[2].
LH (luteinizing hormone)
LH works in concert with FSH. During the reproductive years, a mid-cycle LH surge triggers ovulation. In perimenopause, LH levels tend to rise along with FSH, though less dramatically. The FSH-to-LH ratio can be informative — in perimenopause, FSH typically rises faster than LH, producing an FSH/LH ratio greater than 1[2].
Estradiol (E2)
Estradiol is the primary active estrogen. During perimenopause, estradiol levels become unpredictable. Paradoxically, estradiol can spike to levels higher than normal reproductive levels early in perimenopause — sometimes exceeding 300-400 pg/mL — before eventually declining to the postmenopausal range of under 30 pg/mL[3].
These estradiol spikes (driven by elevated FSH overstimulating remaining follicles) are often responsible for breast tenderness, heavy bleeding, headaches, and mood swings during perimenopause. Consistently low estradiol (below 30-50 pg/mL) combined with elevated FSH (above 40 IU/L) indicates late perimenopause or menopause.
Progesterone
Progesterone is produced by the corpus luteum after ovulation. As cycles become anovulatory (no egg released) during perimenopause, progesterone production drops. A day 21 progesterone below 3 ng/mL suggests the cycle was anovulatory[4].
The loss of progesterone relative to estrogen creates a state of "estrogen dominance" — not because estrogen is necessarily high, but because the counterbalancing effect of progesterone is absent. This can drive heavy or prolonged periods, breast tenderness, anxiety, and sleep disturbance. Progesterone testing helps determine whether anovulation is contributing to symptoms.
AMH (anti-Mullerian hormone)
AMH is produced by small ovarian follicles and directly reflects the remaining follicle pool. Unlike FSH and estradiol, AMH is relatively stable across the menstrual cycle — you can test it any day[5].
AMH declines steadily with age and becomes undetectable approximately 5 years before menopause. An AMH below 1.0 ng/mL indicates significantly diminished ovarian reserve. Below 0.5 ng/mL suggests menopause is likely within 3-4 years. AMH is the single best predictor of time to final menstrual period[6].
Beyond hormones: the tests most doctors forget
Testosterone (total and free)
Testosterone in women declines gradually from the late 20s — it is not solely a menopausal event. By the time a woman reaches her 40s, testosterone is typically 50% of its peak value. Surgical menopause (oophorectomy) causes an immediate 50% further drop because the ovaries produce approximately half of circulating testosterone[7].
Low testosterone contributes to reduced libido, decreased energy, loss of lean muscle mass, and a diminished sense of vitality. Measuring both total testosterone and free testosterone (the unbound, biologically active fraction) gives the full picture. Free testosterone is particularly important because SHBG levels change during perimenopause.
SHBG (sex hormone-binding globulin)
SHBG binds testosterone and estradiol in the bloodstream, reducing their bioavailability. SHBG levels tend to increase after menopause, further reducing the already-declining free testosterone. Weight gain, insulin resistance, and hypothyroidism lower SHBG, while oral estrogen HRT raises it significantly (transdermal estrogen has a much smaller effect on SHBG)[7].
Testing SHBG alongside total hormones allows calculation of free hormone levels and helps explain symptom patterns that total hormone levels alone cannot.
Thyroid panel (TSH, free T4, free T3)
This is arguably the most important "add-on" test during perimenopause. Autoimmune thyroid disease (Hashimoto's) affects approximately 10-15% of women, and incidence increases with age. The symptom overlap between hypothyroidism and perimenopause is extensive: fatigue, weight gain, mood changes, brain fog, hair thinning, and irregular periods[8].
Without thyroid testing, these symptoms may be incorrectly attributed entirely to perimenopause, and treatable thyroid dysfunction goes unaddressed. Test TSH, free T4, and free T3 at minimum. If TSH is borderline (2.5-4.5 mIU/L), add thyroid peroxidase antibodies (TPO) and thyroglobulin antibodies (TgAb) to check for autoimmune thyroid disease.
Metabolic panel
Cardiovascular risk increases significantly during the menopausal transition. Declining estrogen affects lipid metabolism, insulin sensitivity, and vascular function. The SWAN study (Study of Women's Health Across the Nation) showed that total cholesterol, LDL, and triglycerides increase substantially during perimenopause, with the steepest rise occurring in the 1-2 years around the final menstrual period[9].
A baseline metabolic panel should include fasting glucose, fasting insulin (or HOMA-IR), HbA1c, total cholesterol, LDL, HDL, triglycerides, and Lp(a) if not previously measured. Repeating the lipid panel annually through the transition helps catch the predictable rise early.
When to test: timing matters
| Marker | When to test | Why timing matters |
|---|---|---|
| FSH, LH, estradiol | Cycle days 2-5 (if cycling) | Early follicular phase gives the most interpretable baseline |
| Progesterone | Day 21 (or 7 days post-ovulation) | Confirms whether ovulation occurred |
| AMH | Any day | Stable across the cycle — no timing required |
| Testosterone, SHBG | Morning (8-10 AM), any cycle day | Testosterone peaks in the morning |
| Thyroid (TSH, fT4, fT3) | Morning, fasting preferred | TSH follows a diurnal pattern; morning fasting gives most consistent results |
| Metabolic (glucose, insulin, lipids) | Morning, 10-12 hour fast | Fasting required for accurate glucose, insulin, and triglyceride measurement |
If your cycles are irregular or absent: test any time. The variability itself is diagnostically meaningful. Repeat testing in 4-6 weeks if results are ambiguous — seeing two data points is far more useful than one during perimenopause.
Monitoring HRT
If you start hormone replacement therapy, blood work becomes a dosing tool. The goal is to achieve symptom relief with hormone levels in a physiological range[10].
- Estradiol: Test 4-6 weeks after starting or adjusting dose. For transdermal estradiol, test the trough level (just before applying the next patch or dose). Target trough estradiol is typically 50-150 pg/mL for symptom relief, though individual needs vary.
- FSH: Should drop below 30 IU/L with adequate estrogen replacement. If FSH remains elevated, the estrogen dose may be insufficient.
- Progesterone: If taking micronized progesterone (Utrogestan), serum levels are difficult to interpret due to variable absorption. Clinical response (cycle control, sleep improvement) is a better guide than blood levels for oral progesterone[11].
- Testosterone: If on testosterone therapy, test total and free testosterone 3-6 hours after application (for topical) to check peak levels, and at trough (before next dose) to confirm adequate baseline. Target total testosterone for women is typically 30-70 ng/dL.
- Lipids and metabolic markers: Recheck 3-6 months after starting HRT, then annually. Transdermal estrogen tends to be lipid-neutral or mildly beneficial; oral estrogen raises triglycerides[12].
The STRAW staging system: where are you?
The STRAW+10 system divides reproductive aging into stages. Understanding where you are helps frame your blood work expectations[1]:
| Stage | Cycles | FSH (day 2-5) | AMH |
|---|---|---|---|
| Late reproductive | Regular, slightly shorter | Normal or mildly elevated | Low-normal (1.0-2.0 ng/mL) |
| Early perimenopause | Variable length (7+ day shifts) | Elevated (> 25 IU/L), variable | Low (< 1.0 ng/mL) |
| Late perimenopause | Gaps of 60+ days | Elevated (> 25 IU/L), often > 40 | Very low or undetectable |
| Postmenopause | Absent 12+ months | Persistently > 40 IU/L | Undetectable |
Bone density and vitamin D
Estrogen is protective for bone. The most rapid bone loss occurs in the 2-3 years surrounding menopause — women can lose 2-3% of bone mineral density per year during this window[13]. While bone density is measured by DEXA scan (not blood work), several blood markers support bone health monitoring:
- Vitamin D (25-hydroxyvitamin D): Target 40-60 ng/mL. Deficiency is extremely common and accelerates bone loss.
- Calcium: Serum calcium is tightly regulated and not a useful indicator of intake, but ionized calcium can be informative.
- CTX (C-telopeptide): A bone resorption marker. Elevated CTX indicates accelerated bone breakdown.
Mental health: the overlooked connection
The perimenopausal window carries a 2-4x increased risk of depression compared to premenopausal years, even in women with no prior history[14]. Estrogen modulates serotonin, norepinephrine, and dopamine. As estradiol fluctuates wildly during perimenopause, neurotransmitter systems are destabilized.
This is not "just hormones" in a dismissive sense — it is a neurobiological phenomenon with measurable correlates. If you are experiencing new-onset anxiety, depression, or cognitive changes during perimenopause, hormonal blood work combined with thyroid testing can help determine whether hormone-based interventions (including HRT) are appropriate.