Night Sweats — Blood Tests and Causes
Waking drenched in sweat has many explanations — from perimenopause and thyroid dysfunction to infection and, rarely, lymphoma. Here is what the blood tests show and when to act.
The Quick Answer
Night sweats are true drenching sweats that wake you from sleep and are not explained by a hot bedroom or too many blankets. The most common causes in Australian adults are perimenopause and menopause (in women), hyperthyroidism, infection, and medications. A standard blood panel will identify a cause in most cases. The rare but important causes — lymphoma and serious infection — are screened for in the same panel.
Causes of Night Sweats
Night sweats have a long differential. These are the conditions most commonly identified in Australian primary care, grouped by category.
Perimenopause
Fluctuating oestrogen causes the hypothalamic thermostat to misfire, triggering vasodilation and sweating. The most common cause of night sweats in women aged 40–55. FSH, LH, and oestradiol help confirm but may be variable in perimenopause.
Menopause
After 12 months without periods, declining oestrogen causes hot flushes and night sweats in up to 80% of women. FSH is typically elevated above 30 IU/L. Most resolve within 2–5 years but may persist longer.
Hyperthyroidism
Excess thyroid hormone raises metabolic rate, increasing body temperature and sweating. Often accompanied by palpitations, weight loss despite good appetite, anxiety, and tremor. Diagnosed with TSH (suppressed) + elevated free T4/T3.
Infection (TB, endocarditis, abscess)
Classic "drenching" night sweats with fever and weight loss are the hallmark of tuberculosis. Bacterial endocarditis (heart valve infection) and occult abscesses also present this way. CRP, ESR, blood cultures, and chest X-ray are essential if infection is suspected.
Lymphoma and leukaemia
Drenching night sweats are a "B symptom" of Hodgkin and non-Hodgkin lymphoma. Combined with weight loss and swollen lymph nodes, this triad needs urgent assessment. LDH and FBC with film are the initial blood tests; CT scan follows if abnormal.
Low blood sugar (hypoglycaemia)
Overnight glucose drops trigger adrenaline release — sweating, shaking, and waking. Most common in people on insulin or sulfonylureas. HbA1c and fasting glucose are part of the workup; a continuous glucose monitor can detect nocturnal dips.
GERD / acid reflux
Oesophageal irritation from reflux during sleep can trigger autonomic responses including sweating. Often accompanied by heartburn, regurgitation, or a sour taste. Usually diagnosed clinically rather than by blood test.
Anxiety and stress
Chronic anxiety activates the sympathetic nervous system, which controls sweating. Night sweats from anxiety are often accompanied by racing thoughts, nightmares, or difficulty staying asleep. Blood tests are used to rule out physical causes.
Medications
Antidepressants (SSRIs, SNRIs), tamoxifen, opioids, and some antihypertensives cause night sweats as a direct side effect. Review your medication list — timing of sweats relative to starting a new medication is often diagnostic.
Sleep apnoea
Obstructive sleep apnoea causes repeated overnight oxygen dips and autonomic surges that may trigger sweating episodes. Often accompanied by snoring, witnessed apnoeas, and daytime fatigue. Diagnosed with a sleep study, not blood tests.
Alcohol withdrawal
Heavy regular drinkers who reduce or stop alcohol experience autonomic instability including sweating, tremor, and anxiety, often worst overnight. This can be medically serious and requires GP supervision.
When to See Your GP About Night Sweats
Not every episode of overnight sweating warrants investigation. But certain patterns always do:
- Sweats that wake you from sleep more than once a week for longer than 2–3 weeks
- Sweats that soak through pyjamas or require changing the sheets
- Any combination of sweats with fever, weight loss, or swollen glands
- Night sweats in a man of any age (less common physiologically, more likely to have a pathological cause)
- Night sweats that began at the same time as a new medication
- Night sweats with palpitations, tremor, anxiety, or weight loss (thyroid screen needed)
- Night sweats with shaking, confusion, or waking hungry (nocturnal hypoglycaemia screen needed)
Blood Tests for Night Sweats
| Test | Why It Is Ordered | What It Detects |
|---|---|---|
| TSH (Thyroid-Stimulating Hormone) | First-line thyroid screen | Hyperthyroidism (suppressed TSH), hypothyroidism (elevated TSH) |
| Free T4 | Confirms hyperthyroidism if TSH suppressed | Elevated in hyperthyroidism; low in hypothyroidism |
| Free T3 | Detects T3 toxicosis (TSH suppressed, T4 normal) | Isolated T3 elevation in some hyperthyroid cases |
| FSH (Follicle-Stimulating Hormone) | Menopause / perimenopause assessment | Elevated FSH (>30 IU/L) suggests menopause; variable in perimenopause |
| Oestradiol | Oestrogen level for menopause workup | Declining oestradiol confirms ovarian function loss |
| LH (Luteinising Hormone) | Paired with FSH for menopause assessment | Elevated LH alongside FSH confirms hypothalamic response to low oestrogen |
| Full Blood Count (FBC) + film | Broad screen for infection, anaemia, haematological malignancy | Lymphocytosis (lymphoma/leukaemia), anaemia, thrombocytopenia, blast cells |
| ESR (Erythrocyte Sedimentation Rate) | Systemic inflammation screen | Elevated in infection, lymphoma, autoimmune disease, TB |
| CRP (C-Reactive Protein) | Acute-phase inflammation marker | Elevated in active infection, inflammatory disease; very high in bacterial infections |
| LDH (Lactate Dehydrogenase) | Lymphoma and tissue damage screen | Elevated in lymphoma, haemolysis, heart attack, liver disease; non-specific but useful |
| Blood cultures (if febrile) | Identifies bloodstream infection | Bacteraemia, endocarditis, sepsis |
| HbA1c + fasting glucose | Diabetes and blood sugar control | Elevated HbA1c suggests insulin resistance; used alongside glucose for nocturnal hypoglycaemia context |
| HIV serology | HIV causes night sweats, weight loss, and opportunistic infections | HIV infection (offered based on risk factors and clinical picture) |
How Your GP Will Investigate Night Sweats
Characterise the sweats
Your GP will ask: Are they drenching (soaking through bedclothes) or mild? How often? How long have they been going on? Any associated fever, weight loss, or lumps? Any new medications? The pattern often points directly to the category of cause.
Hormonal screen (women under 60)
FSH, LH, and oestradiol to assess for perimenopause or menopause. In women still having periods, the test timing in the cycle affects results. Your GP may repeat tests at different cycle stages if the first result is equivocal.
Thyroid function
TSH is the primary screen. If suppressed, free T4 and free T3 are added to confirm hyperthyroidism and quantify severity. Thyroid antibodies (anti-TPO) may be checked if autoimmune thyroid disease is suspected.
Infection and inflammation screen
FBC, CRP, and ESR form the basic inflammation screen. If TB is a concern (overseas travel, contact history, immunosuppression), a chest X-ray and Mantoux or IGRA test may be added. Blood cultures are collected if there is fever.
Lymphoma screen
LDH is included in the standard workup. A blood film is reviewed for abnormal lymphocytes. If lymphadenopathy (swollen glands) is present or LDH is elevated, CT scan and haematology referral are arranged.
Metabolic causes
HbA1c and fasting glucose screen for diabetes. If nocturnal hypoglycaemia is suspected in a diabetic patient, a continuous glucose monitor worn overnight is the most informative investigation.
Review medications
A careful medication and supplement review often reveals the cause, particularly for antidepressant-related sweats. Timing relative to starting a new drug is often diagnostic. Your GP can advise on dose adjustment or switching.
Red Flags — Act Promptly
Night sweats + unexplained weight loss + painless lumps
The classic lymphoma triad ("B symptoms" of fever, drenching sweats, and >10% weight loss in 6 months). Needs FBC, LDH, CT scan, and haematology referral.
Drenching sweats + persistent fever
Suggests active infection — TB, endocarditis, abscess, or other serious bacterial infection. Blood cultures, CRP, and chest X-ray are urgently needed. Do not wait.
Night sweats + waking confused or shaky
Pattern of nocturnal hypoglycaemia, especially in diabetics on insulin or sulfonylureas. Severe overnight hypoglycaemia is dangerous. Check glucose and review diabetes medications urgently.
Night sweats + palpitations + weight loss
Suggests hyperthyroidism. TSH is the initial test. If thyrotoxicosis is confirmed, treatment can prevent serious cardiac complications.
Night sweats after starting a new medication
Drug-induced sweats are common and usually resolve with dose adjustment or switching. Do not stop medications suddenly without GP guidance, especially antidepressants.
Practical Steps While You Await Results
Bedroom temperature
Keep the bedroom cool (16–19°C). Use breathable cotton or bamboo bedding rather than synthetic materials, which trap heat. A fan can help significantly.
Alcohol before bed
Even moderate alcohol before sleep raises core body temperature in the second half of the night, triggering sweats independent of any underlying condition. Avoiding alcohol within 3 hours of sleep often produces immediate improvement.
Spicy foods and caffeine
Both can trigger vasodilation and sweating. Avoiding spicy meals and caffeine after midday reduces overnight sweating episodes for many people regardless of the underlying cause.
Stress management
Chronic anxiety and stress activate the sympathetic nervous system continuously. Regular aerobic exercise, mindfulness practice, and adequate sleep hygiene reduce background sympathetic tone and sweating frequency.
Weight management
Excess weight is an independent contributor to both night sweats and menopausal symptoms. Even modest weight loss (5–10%) can meaningfully reduce hot flushes and night sweating frequency.
Layer bedding for easy adjustment
Using a light sheet with a separate duvet allows quick temperature adjustment during the night without fully waking. This does not address the cause but reduces sleep disruption while you investigate.
Related Reading
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This page provides general educational information about night sweats and their potential causes. It is not a substitute for professional medical advice, diagnosis, or treatment. Persistent night sweats — particularly with weight loss, fever, or lymph node swelling — require prompt GP assessment. SmarterBlood does not provide medical care.