THE SCIENCE OF HORMONES
How hormones shape every decade of life.
Hormones are chemical messengers that coordinate temperature, sleep, mood, bone, muscle, metabolism and sexual health. Here is what changes with age, what the transition to menopause actually looks like, and what the evidence says about timing and treatment.
THE MESSENGERS
Four hormones, most of the story.
Dozens of hormones matter for midlife health, but four explain most of what changes and most of what we treat.
MADE IN THE OVARIES
Estradiol
The main estrogen before menopause. It regulates the menstrual cycle and acts on the brain’s temperature center, bone, blood vessels, skin and genitourinary tissue.
Temperature · bone · vessels · GU tissue
MADE AFTER OVULATION
Progesterone
Prepares and protects the uterine lining. Because ovulation becomes irregular first, falling progesterone is often the earliest hormonal change of perimenopause.
Uterine lining · sleep · cycle
TESTES · OVARIES · ADRENALS
Testosterone
Supports muscle, bone, red blood cell production and libido in men and women. Part of it is converted into estradiol, which men also need.
Muscle · bone · blood · libido
MADE IN THE THYROID
T4 and T3
Released under the control of TSH from the pituitary. T4 is converted into active T3 in tissues, setting metabolic rate, heart rhythm and body temperature.
Metabolism · heart · energy
ACROSS THE LIFESPAN
How hormones change, decade by decade.
Switch between women and men and drag the slider. The shapes are illustrative, but the pattern is real: estradiol and progesterone fall over a few years around menopause, while testosterone in men declines slowly over decades.
Illustrative shapes of typical patterns, not laboratory reference ranges. Monthly cycles are compressed for display. Drag the slider to explore each life stage.
THE MENOPAUSE TRANSITION
The stages, as clinicians define them.
The STRAW+10 staging system defines the transition by bleeding patterns, not by a blood test. Hormone levels swing too much in perimenopause for a single result to confirm or rule it out.
STAGE −2
Early transition
Cycle length varies by 7 days or more between consecutive cycles, and the change persists. FSH is raised but variable.
STAGE −1
Late transition
60 days or more without a period. Hot flashes are most likely to begin here. This stage usually lasts one to three years.
STAGE 0
Final menstrual period
Confirmed only in hindsight, after 12 months without a period. The average age in the US is about 51.
STAGE +1 / +2
Postmenopause
Bone loss is fastest in the first years. Later, genitourinary symptoms tend to progress without treatment.
Source: Harlow SD et al. Executive summary of the Stages of Reproductive Aging Workshop + 10. Menopause 2012;19(4):387–395.
THE TIMING HYPOTHESIS
Age and timing change the math.
The Women’s Health Initiative enrolled women who averaged 63 years old. Later analyses and trials suggest that starting hormone therapy closer to menopause carries a more favorable balance of benefit and risk.
For bothersome hot flashes or to prevent bone loss, and with no contraindications, The Menopause Society (2022) considers the benefit-risk ratio favorable for women under 60 or within 10 years of menopause. Updated FDA labeling (2025) also recommends starting systemic therapy in this window.
At any age, a history of breast or endometrial cancer, unexplained vaginal bleeding, blood clots, stroke, heart disease or active liver disease needs specialist review first.
Educational tool, not medical advice. Sources: The Menopause Society 2022 Hormone Therapy Position Statement; FDA labeling update, November 2025.
ELITE TRIAL
Earlier start, slower artery thickening
Oral estradiol slowed progression of carotid artery wall thickening in women who started within 6 years of menopause, but not in women who started 10 or more years after.
Hodis HN et al. N Engl J Med 2016;374:1221–1231.
WHAT IT DOES NOT MEAN
Not a reason to start for prevention alone
Major societies do not recommend hormone therapy solely to prevent heart disease or dementia. The timing window describes when treating symptoms is safest, not a new indication.
The Menopause Society 2022 Hormone Therapy Position Statement.
MEN’S HORMONES
Testosterone: what the evidence supports.
Testosterone falls slowly with age, and many older men with low-normal levels do not have hypogonadism. Diagnosis needs consistent symptoms and repeatedly low early-morning levels. Treatment decisions weigh fertility, red blood cell count, prostate health and cardiovascular risk.
Sources: Lincoff AM et al. NEJM 2023 (TRAVERSE); Snyder PJ et al. NEJM 2024 (TRAVERSE fracture substudy).
0.96
hazard ratio for heart attack, stroke or cardiovascular death on testosterone vs placebo (TRAVERSE)
1.43
hazard ratio for clinical fracture on testosterone vs placebo in the TRAVERSE fracture substudy
WHAT WE MEASURE
The labs and scans that actually matter.
01
Estradiol and FSH
Swing widely in perimenopause, so one result cannot confirm or rule it out. Over 45, the diagnosis is usually clinical.
02
Morning testosterone
Total testosterone on two separate early mornings, with free testosterone or SHBG when results are borderline.
03
TSH and free T4
First-line thyroid tests. Thyroid symptoms often overlap with menopause, so we check before attributing them.
04
DXA bone density
Recommended for women 65 and older, and for younger postmenopausal women at increased fracture risk (USPSTF).
05
Lipids and ApoB
LDL cholesterol and ApoB often rise after menopause and guide cardiovascular prevention.
06
Hematocrit and PSA
Monitored in men on testosterone. The Endocrine Society advises pausing treatment if hematocrit rises above 54%.