How the endocrine system works
Hormones are produced by glands (thyroid, adrenals, pancreas, ovaries, testes, pituitary) and circulate through the blood to receptors across the body. The system runs largely on feedback: the pituitary senses the effect and adjusts by releasing more or less stimulating hormone. Small disruptions in that loop tend to produce large, broad complaints.
The four main axes that affect you directly: the thyroid axis (metabolism), the HPA axis (cortisol, stress response), the gonadal axis (testosterone, oestrogen, progesterone) and the insulin response. Disruptions in one axis often cascade into the others — untreated hypothyroidism raises cholesterol; chronic stress lowers testosterone.
Thyroid
The thyroid sets the tempo at which your body runs. Underactive (hypothyroidism) causes fatigue, weight gain, cold intolerance, dry skin, depressive symptoms; overactive (hyperthyroidism) causes palpitations, weight loss, heat intolerance, inner restlessness. Subclinical forms — TSH mildly off, fT4 still normal — are often symptomatic but easy to miss.6
First-line measurement is TSH; if TSH is off, free T4 follows (and sometimes T3). The Optimize Baseline measures TSH by default and our triage system flags values outside the optimal range. For deeper context, see the Energy guide.
Cortisol and the adrenals
Cortisol — often called 'the stress hormone' — follows a natural daily curve: peaks early in the morning, drops across the day, lowest around bedtime. That curve keeps you alert by day and lets you wind down at night. Chronic stress flattens the curve: lower morning peak, higher evening cortisol, with disturbed sleep and metabolic effects as a result.
A single blood cortisol says relatively little about chronic stress. Salivary cortisol at multiple time points (the curve) or hair cortisol (chronic exposure over weeks–months) give a richer picture. For deeper context, see the Stress guide.
For men — testosterone across the lifespan
Testosterone peaks in the early twenties and from around age 35-40 falls on average ~1% per year, with wide individual variation.2,3 A diagnosis of hypogonadism requires symptoms *plus* repeatedly low morning testosterone — total below ~8 nmol/L is a common threshold, with a grey zone between 8 and 12 nmol/L. Not either alone.1 In that grey zone, calculating free testosterone via SHBG gives a more reliable picture.
Classic symptoms: low libido, fatigue, reduced muscle mass, loss of morning erections, low mood. But the same symptoms can come from poor sleep, excess weight, depression or an underactive thyroid — so checking those basics first is the sensible order. Poor sleep, alcohol, opioids and obesity measurably lower testosterone; removing those drivers is often more effective than supplementing.
Testosterone therapy (TRT) measurably improves libido, muscle, energy and mood when deficiency is real. But it's a lifelong treatment that shuts down your own production, requires regular monitoring (haematocrit, PSA, lipids) and isn't without risk. It's frequently prescribed to men with symptoms but normal values — a nuance the Endocrine Society guideline makes explicit.1 A conversation with an endocrinologist or urologist belongs in the decision.
For women — cycle, peri- and menopause
In women of reproductive age, oestrogen, progesterone, LH and FSH fluctuate across a ~28-day cycle. A single measurement at one cycle point says little — a hormone panel requires a draw on a specific cycle day or interpretation becomes unreliable. Complaints often track the phase: PMS or PMDD in the luteal phase (after ovulation), heavy bleeding or pain around menstruation. For cycle complaints that affect daily life, a gynaecologist is the right route.
Perimenopause — the years before menopause — often starts around 40-45 and can last a decade. The cycle becomes irregular, FSH rises and oestrogen fluctuates sharply. Many women in this phase already experience hot flushes, sleep disturbance, mood swings, brain fog and weight gain — while routine bloodwork still reads 'normal' because the values swing day to day. A single normal FSH therefore doesn't rule perimenopause out.
Menopause (median 51 years) is defined as 12 months without a period. Oestrogen falls permanently, with hot flushes, sleep disturbance, genitourinary symptoms, and over the longer term raised risk of bone loss and cardiovascular disease. The NAMS 2022 position statement is clear: for most women under 60 or within 10 years of menopause, the benefit of hormone therapy (HT) outweighs the risk — particularly for moderate to severe symptoms.7,8 This is a conversation with a gynaecologist or menopause specialist, not a DIY route.
Women also produce testosterone — mainly via the ovaries and adrenals — and the values matter for libido, energy and muscle mass. The Endocrine Society advises against routine testing in women without clear suspicion but acknowledges a role in hypoactive sexual desire disorder after menopause.4,5 Testosterone therapy in women is always specialist-led and in many European countries off-label.
What to measure and when
Baseline measurements in the Optimize Baseline: TSH (thyroid) and HbA1c (insulin response over 3 months). For targeted questions we can offer extended panels: in men, morning testosterone (total and free), SHBG, LH and sometimes oestradiol; in women — depending on life stage — FSH, oestradiol, progesterone on the correct cycle day, SHBG and testosterone.
Timing of the blood draw matters a lot. Testosterone is measured in the morning (between 8 and 10am, fasting). Cycle hormones on a specific cycle day. Cortisol in the morning (between 7 and 9am). Outside those windows interpretation gets unreliable — our physician helps pick the right test at the right time.
What you can do yourself
Hormones are largely the product of lifestyle. The four levers with the most impact: sleep, resistance training, nutrition (particularly enough protein) and stress management. A few short nights measurably lower testosterone; chronic stress flattens the cortisol curve; insufficient protein and resistance training accelerates muscle loss around and after menopause or andropause.
Practically: fixed sleep times and morning daylight (see the Sleep guide), two resistance sessions per week (see the Exercise guide), 1.2-1.6 g of protein per kg body weight per day spread across meals (see the Nutrition guide), and taking chronic stress seriously (see the Stress guide). Hormone supplementation — testosterone, menopausal hormone therapy — can be useful in specific situations, but works best once those basics are in place and always under specialist supervision.
When to involve a physician
Book a GP or endocrinologist appointment for persistent fatigue with other hormonal symptoms, unexplained weight loss or gain, markedly reduced libido, irregular or severely painful periods, post-menopausal bleeding, palpitations or anxiety, or symptoms that affect daily functioning. Hormonal treatments — testosterone replacement, menopausal hormone therapy, thyroid medication — require specialist supervision and regular monitoring. At Optimize every result is reviewed by our physician and we reach out proactively when values warrant follow-up.