Perimenopause symptoms: what's changing and how to check your hormones

Declining and fluctuating oestrogen levels

The central driver of perimenopause is a gradual decline in oestrogen (specifically oestradiol), produced as the ovaries become less responsive to the pituitary's signalling hormones. What makes perimenopause particularly confusing is that oestrogen does not fall in a smooth, linear way — it fluctuates dramatically, sometimes spiking higher than premenopausal levels before dropping. This erratic pattern means that a single blood test taken on any given day may show oestradiol in a normal or even elevated range, even in someone who is clearly perimenopausal. The fluctuation itself is what produces many of the symptoms: hot flushes, night sweats, and mood instability are often triggered by the rate of oestrogen change rather than absolute levels.

Rising FSH as the brain tries to compensate

As the ovaries become less responsive, the pituitary gland increases its output of follicle-stimulating hormone (FSH) in an attempt to stimulate egg development. Consistently elevated FSH — particularly above 25 IU/L — can suggest that the ovaries are losing their responsiveness. However, FSH levels fluctuate significantly in perimenopause and a single normal result does not exclude perimenopause. For women under 40 experiencing possible perimenopause symptoms, two elevated FSH results taken four to six weeks apart are used to support a diagnosis of premature ovarian insufficiency (POI).

Declining progesterone and cycle changes

Progesterone, produced in the second half of the menstrual cycle after ovulation, is often the first hormone to fall in early perimenopause. As ovulation becomes less consistent, progesterone output in the luteal phase declines, leading to shorter cycles, heavier or irregular periods, more pronounced PMS, and worsening sleep quality. Progesterone also has an anxiolytic (anxiety-reducing) effect via its conversion to allopregnanolone, which acts on GABA receptors. Declining progesterone can therefore contribute directly to increased anxiety, poor sleep, and mood changes — often before oestrogen levels have shifted significantly.

The thyroid-perimenopause overlap

Thyroid conditions become more common in women during their 40s and produce symptoms that closely mirror perimenopause: fatigue, weight changes, mood shifts, difficulty sleeping, brain fog, and altered periods. An underactive thyroid slows these processes; an overactive thyroid accelerates them. Distinguishing between thyroid dysfunction and perimenopause — or identifying that both are present at once — requires testing both sets of markers. Thyroid function should always be assessed alongside reproductive hormones in any perimenopausal investigation.

Testosterone decline and its effects

Testosterone is produced in the ovaries and adrenal glands throughout a woman's life and declines gradually from the mid-30s onwards. Low testosterone in women is associated with reduced libido, loss of motivation, difficulty concentrating, and a general reduction in energy and drive that is distinct from the fatigue caused by oestrogen changes. In perimenopause, declining testosterone can compound the effects of falling oestrogen — but it is rarely tested as part of a standard NHS assessment.

Brain fog, memory, and the cognitive effects of hormonal change

Cognitive symptoms are among the most distressing and least validated aspects of perimenopause. Many women experience difficulty with word retrieval, concentration, and short-term memory during perimenopause, and find these dismissed as anxiety or early dementia. The relationship between oestrogen and cognitive function is well-established: oestradiol supports neuronal health, blood flow to the brain, and the function of neurotransmitter systems including serotonin and dopamine. The fluctuating oestrogen of perimenopause disrupts these systems, producing real cognitive symptoms that typically improve as oestrogen stabilises — either naturally at menopause or with HRT.

The British Menopause Society's position is that a blood test is not required to diagnose perimenopause in women aged 45-55 who have typical symptoms — in this age group, the diagnosis is primarily clinical. However, testing is valuable for several important reasons: to rule out conditions that mimic perimenopause (especially thyroid disorders and anaemia), to investigate symptoms in women under 45 where the picture is less clear, and to establish a baseline of metabolic and hormonal markers before any treatment decisions are made.

FSH (follicle-stimulating hormone) rises as the ovaries become less responsive. A consistently elevated FSH supports a perimenopausal or menopausal picture. However, because FSH fluctuates so significantly during perimenopause, a single result — even a normal one — does not rule it out.

Oestradiol reflects the level of the most bioactive oestrogen at the moment the blood was taken. Because it fluctuates dramatically throughout the perimenopausal transition, a single measurement is of limited value in confirming or denying perimenopause. It is most useful when monitoring HRT absorption once treatment is established.

TSH and Free T4 assess thyroid function. Thyroid conditions that closely mimic perimenopausal symptoms are common in this age group and should be tested in any perimenopausal investigation, even if symptoms appear straightforwardly hormonal.

Ferritin and a full blood count assess whether iron deficiency anaemia — a very common cause of fatigue, brain fog, and low mood, particularly in women with heavier periods — is contributing to symptoms. Heavy or irregular bleeding during perimenopause frequently causes iron depletion.

Vitamin D should be checked as part of any midlife health assessment. Oestrogen supports vitamin D metabolism and its protective effects on bone. As oestrogen falls, ensuring vitamin D is optimal becomes more important for bone density, immune function, and mood.

Cholesterol and lipid markers change significantly around perimenopause: oestrogen has a cardioprotective effect on lipid profiles, and as it falls, LDL cholesterol typically rises and HDL tends to fall. Establishing a lipid baseline in the perimenopausal transition is important context for long-term cardiovascular health monitoring.

For women who need reproductive hormone testing specifically — FSH, LH, and oestradiol — this is available through your GP or private clinics. Note that for women with regular periods, reproductive hormones should ideally be tested on days 2-5 of the cycle for the most interpretable results. For women with irregular periods, testing at any time alongside repeat measurements gives a more reliable picture.

Nutrition and bone protection before oestrogen falls further

Oestrogen has a direct protective effect on bone density. As oestrogen declines in perimenopause, bone loss accelerates — typically at its fastest rate in the two to three years around the final period. Ensuring adequate calcium (through dairy, fortified plant milks, tinned fish with bones, and leafy greens) and optimal vitamin D (through sun exposure, diet, and supplementation where needed) provides the nutritional foundation for bone maintenance. Tracking vitamin D specifically gives you the data to know whether you're in the range that actually supports bone protection, rather than guessing.

Exercise as metabolic and bone protection

Weight-bearing exercise — walking, running, dancing, resistance training — is among the most well-evidenced interventions for maintaining bone density during perimenopause. Resistance training in particular supports both bone and muscle mass, countering the simultaneous loss of both that occurs as oestrogen falls. Regular physical activity also improves sleep quality, cardiovascular metabolic markers, and mood — all domains affected by the perimenopause transition.

Sleep, cortisol, and the hormonal cascade

Poor sleep during perimenopause is often driven by night sweats (linked to oestrogen fluctuation) but is amplified by elevated cortisol from disrupted sleep itself — creating a cycle where poor sleep raises cortisol, which further disrupts sleep architecture. Managing sleep hygiene, reducing late-day stimulants, and establishing consistent sleep timing help interrupt this cycle. Tracking inflammatory markers (CRP) and metabolic markers (HbA1c) over time shows whether sleep disruption is leaving measurable traces in the biology.

Gut health and oestrogen recirculation

The gut microbiome plays a role in oestrogen metabolism via a collection of bacteria collectively called the estrobolome, which produce enzymes that deconjugate oestrogen, allowing it to be reabsorbed rather than excreted. A disrupted microbiome can impair this process, reducing the body's effective oestrogen activity. This connection between gut health and hormonal balance is an emerging area of research — and one reason why gut health becomes increasingly relevant as a modifiable factor during perimenopause.