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 4.5 mIU/L confirms hypothyroidism. Treatment with levothyroxine usually reverses some — but not all — of the weight gain.
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?