Symptom Guide

Unexplained Weight Gain: What Blood Tests Reveal

If you're gaining weight despite no significant change in diet or exercise, a medical cause may be responsible. These blood tests can identify it.¹

Written by Suman Konda, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last updated: 31 August 2026 · How we check our content

Quick answer

Most weight gain is energy imbalance, but rapid or steady gain without dietary change deserves a medical look. Hypothyroidism is the commonest hormonal cause: unexplained gain of 5 to 10 kg over months with TSH above 4.0. Insulin resistance causes abdominal weight gain. PCOS drives weight gain through insulin resistance and androgens.

Cause group	Clues	Urgency
Hypothyroidism	5 to 10 kg unexplained gain over months, fatigue, cold intolerance; TSH above 4.0 mIU/L	Routine workup
Insulin resistance	Abdominal weight gain; fasting insulin above 15 with normal or borderline sugar	Routine workup
PCOS	Abdominal gain often resistant to dieting; test LH:FSH, testosterone, AMH	Routine workup
Unexplained rapid gain	Steady gain with no change in diet or exercise	Seek prompt care

Key points

Most weight gain is energy imbalance; rapid or steady unexplained gain needs medical assessment.
Hypothyroidism: 5 to 10 kg over months, TSH above 4.0 confirms.
Insulin resistance: abdominal fat, fasting insulin above 15 with normal or borderline sugar.
PCOS: abdominal gain often resistant to dieting without addressing hormones; test LH:FSH, testosterone, AMH.
BMI does not show where fat sits; central (belly) fat carries the metabolic risk.

Medical causes of unexplained weight gain

Most weight gain is from energy imbalance: but when weight increases rapidly or steadily despite no dietary change, blood tests should look for these conditions:

1. TSH (Thyroid): Most important first test

Hypothyroidism (underactive thyroid) is the most common hormonal cause of weight gain. A slow thyroid reduces metabolism: everything slows down, including calorie burning. Weight gain of 5–10 kg over months without explanation is a classic hypothyroid symptom. TSH above 4.0 mIU/L confirms hypothyroidism. Treatment with levothyroxine usually reverses some, but not all, of the weight gain. The size of the effect is easy to overstate, though. Where the thyroid is only mildly underactive – a raised TSH with a normal free T4 – a placebo-controlled trial in 737 adults aged 65 and over found levothyroxine normalised TSH without improving hypothyroid symptoms or tiredness.² A borderline TSH is therefore rarely the whole explanation for substantial weight gain, and treating the number alone tends to disappoint.

2. Fasting Insulin + Blood Sugar: Insulin resistance

Insulin resistance (pre-diabetes) causes weight gain, especially around the abdomen. High insulin promotes fat storage. Fasting insulin above 15 µIU/mL with a normal or borderline fasting sugar = insulin resistance. This is extremely common in people with central obesity, PCOS, and family history of diabetes.

3. Hormone Panel for Women (PCOD/PCOS)

PCOS causes weight gain through insulin resistance and elevated androgens. LH:FSH ratio, testosterone, and AMH are the key tests. Weight gain in PCOS tends to be abdominal and is often resistant to dieting without addressing the underlying hormonal imbalance.

4. Cortisol (for Cushing's Syndrome)

Very high cortisol (Cushing's syndrome) causes rapid weight gain specifically in the face (moon face), upper back (buffalo hump) and abdomen, with thin arms and legs. Rare but important to rule out. A 24-hour urine cortisol or overnight dexamethasone suppression test screens for this.

5. Oedema causes: KFT, LFT, albumin

Sometimes apparent weight gain is actually fluid retention (oedema). Kidney disease (low albumin, protein loss), liver disease (cirrhosis, low albumin) and heart failure all cause fluid accumulation in tissues. If ankles and legs are swollen, check kidney and liver function tests.

Questions to ask your doctor

Is my TSH normal, even a TSH of 3–4 mIU/L can cause symptoms in some people?
Should I check fasting insulin to look for insulin resistance?
Could my medication (antidepressants, steroids, antipsychotics) be causing the weight gain?
Is any of this weight fluid retention rather than fat?

References

Sources cited on this page. PubMed links open the original abstract.

Bray GA, Kim KK, Wilding JPH; World Obesity Federation. Obesity: a chronic relapsing progressive disease process. Obes Rev. 2017;18(7):715–723. PMID 28489290 · doi:10.1111/obr.12551
Stott DJ, Rodondi N, Kearney PM, et al. Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism. N Engl J Med. 2017;376(26):2534–2544. PMID 28402245 · doi:10.1056/NEJMoa1603825
WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157–163. PMID 14726171 · doi:10.1016/S0140-6736(03)15268-3

Medical causes that blood tests can identify

Most weight gain is explained by caloric excess, reduced activity, or medication side effects. However, these specific conditions cause weight gain through distinct metabolic mechanisms and are diagnosable with blood tests:

Hypothyroidism: An underactive thyroid slows basal metabolic rate. Weight gain is typically modest (2–5 kg) and accompanied by fatigue, cold intolerance, dry skin, constipation, and slow heart rate. TSH is the screening test, a raised TSH (above 4.0 mU/L) with a low free T4 confirms primary hypothyroidism. Completely reversible with levothyroxine replacement.
Polycystic ovary syndrome (PCOS): The most common hormonal disorder in women of reproductive age. Insulin resistance drives weight gain, particularly abdominal fat. Blood tests: raised LH:FSH ratio, raised androgens (testosterone, DHEAS), fasting insulin, and HbA1c. PCOS is a risk factor for type 2 diabetes and cardiovascular disease.
Cushing's syndrome: Excess cortisol (from a pituitary tumour, adrenal tumour, or long-term steroid treatment) causes characteristic weight gain concentrated on the face (moon face), trunk (buffalo hump), and abdomen, with thin limbs. Screening: 24-hour urinary free cortisol, overnight dexamethasone suppression test, or late-night salivary cortisol. Rare but important to exclude when the pattern is characteristic.
Insulin resistance / pre-diabetes: Hyperinsulinaemia promotes fat storage. HbA1c between 42–47 mmol/mol (or fasting glucose 6.1–6.9 mmol/L) indicates pre-diabetes. This stage is reversible with lifestyle change, the single most evidence-based intervention for preventing type 2 diabetes.
Medication-related: Corticosteroids, certain antipsychotics (olanzapine, quetiapine), antidepressants, insulin, sulfonylureas, beta-blockers, and some anticonvulsants cause significant weight gain. Review of the medication list is an essential step before extensive endocrine investigation.

What BMI doesn't tell you, and what does

Body mass index (weight in kg ÷ height in metres²) classifies obesity into grades but is a poor predictor of metabolic risk in individuals. Waist circumference and waist-to-hip ratio better predict cardiovascular and metabolic risk, particularly in people of South Asian origin, where obesity-related metabolic risk occurs at a lower BMI. NICE guidance recommends metabolic investigation (glucose, lipids, liver function) in anyone with a BMI above 30, or above 27.5 in people of South Asian descent, a threshold based on a WHO expert consultation identifying 27.5 kg/m² as a key public health action point for Asian populations, whose diabetes and cardiovascular risk rises at lower BMIs than in European populations.³

Medical Disclaimer: This page is for general educational purposes only. It does not constitute medical advice. Always consult a qualified doctor before making any health decisions.