Blood Test for Unexplained Weight Gain

You haven't changed what you eat. You're still exercising. But the scales keep creeping up, and your clothes are getting tighter. Unexplained weight gain — the kind that happens without any obvious change in lifestyle — is almost always driven by something measurable in blood. The question is whether anyone bothers to check.

If your GP has shrugged and suggested eating less, it's time to look deeper. Here's what a blood test can reveal about why you're gaining weight despite doing everything right.

The Thyroid-Weight Connection

Hypothyroidism — an underactive thyroid — is the most common medical cause of unexplained weight gain. It affects around 2% of the UK population, with women five to ten times more likely to be affected than men. The thyroid gland controls your basal metabolic rate: when it slows down, you burn fewer calories at rest, retain more fluid, and gain weight even on a calorie-controlled diet.

The frustrating part is that many people with thyroid-driven weight gain have "subclinical" hypothyroidism — their TSH is mildly elevated (say 4.0–10.0 mIU/L) but still within the NHS reference range of 0.27–4.2. Some labs use an upper limit of 5.0 or even 5.5. This means they're told their thyroid is fine when it isn't functioning optimally.

Why You Need the Full Thyroid Panel

TSH alone doesn't give the full picture. You need:

• TSH — is the pituitary gland asking the thyroid to work harder?
• Free T4 (FT4) — is the thyroid producing enough inactive hormone?
• Free T3 (FT3) — is T4 being converted into the active hormone that drives metabolism?

Some people have normal TSH and FT4 but low FT3 — a conversion problem that directly impairs metabolism. Others have elevated thyroid antibodies (anti-TPO) indicating Hashimoto's thyroiditis, where the immune system gradually destroys the thyroid gland. Hashimoto's can cause weight gain years before TSH rises high enough for a formal diagnosis.

Insulin Resistance: Pre-Diabetes Without the Label

Insulin resistance is arguably more responsible for weight gain in the UK than thyroid dysfunction, but it's tested for far less frequently. An estimated 13.6 million people in the UK are at increased risk of type 2 diabetes, and insulin resistance is the precursor stage that can persist for a decade before blood glucose rises enough for a diabetes diagnosis.

Here's how it works: your cells become less responsive to insulin, so your pancreas produces more and more of it to keep blood glucose in check. High insulin levels promote fat storage — particularly around the abdomen — and make it extremely difficult to lose weight even with calorie restriction. You can be insulin resistant with a perfectly normal fasting glucose.

What to Test

• HbA1c — your 2–3 month average blood glucose. Pre-diabetes is 42–47 mmol/mol. Even levels in the high 30s may indicate early insulin resistance.
• Fasting glucose — useful alongside HbA1c but can be normal in early insulin resistance
• Fasting insulin — the most sensitive early marker. If your insulin is elevated but glucose is still normal, your body is working overtime to compensate. Available as an add-on test.

If your HbA1c is creeping above 38 mmol/mol and you're gaining weight around your middle, insulin resistance is the likely culprit — and it's entirely reversible with the right dietary and lifestyle changes.

Cortisol: The Stress Hormone

Chronic stress elevates cortisol, which promotes visceral fat storage (the deep fat around your organs), increases appetite, and disrupts sleep — all of which drive weight gain. While Cushing's syndrome (pathologically high cortisol) is rare, mildly elevated cortisol from chronic stress, poor sleep, or overtraining is common and contributes to stubborn weight.

Morning cortisol can be measured in blood. Levels consistently above 500 nmol/L in a morning sample, or below 200 nmol/L, warrant further investigation. Cortisol is available as an add-on test with Lola Health panels.

Hormonal Imbalance

Women: Oestrogen Dominance and PCOS

In women, weight gain — particularly around the hips, thighs, and lower abdomen — is often hormonally driven. Two common patterns:

• PCOS — elevated androgens (testosterone) with insulin resistance. Causes central weight gain, acne, irregular periods. Affects 1 in 10 women in the UK.
• Perimenopause and menopause — declining oestrogen shifts fat distribution toward the abdomen and reduces metabolic rate. Most women gain 5–10 lbs during the menopausal transition.

Key markers: testosterone, SHBG, free testosterone, oestradiol, FSH, LH, DHEA-S.

Men: Low Testosterone

Testosterone in men promotes lean muscle mass and helps regulate fat distribution. When testosterone drops — due to ageing, stress, poor sleep, excess body fat, or medical conditions — men lose muscle and gain fat, particularly around the abdomen. This creates a vicious cycle: more body fat produces more aromatase enzyme, which converts testosterone to oestrogen, further lowering testosterone.

Total testosterone below 12 nmol/L is considered low by most UK guidelines. Free testosterone — calculated from total testosterone and SHBG — is often a more useful measure, as high SHBG can mask low free testosterone behind a "normal" total number.

The Lipid Panel: Consequence and Clue

While elevated cholesterol and triglycerides are often a consequence of weight gain, they can also serve as diagnostic clues. Very high triglycerides (above 2.3 mmol/L) alongside low HDL cholesterol strongly suggest insulin resistance. A lipid panel provides context for the metabolic picture and helps assess cardiovascular risk.

Which Blood Test to Order

If thyroid dysfunction is your primary concern, the Thyroid & Hormonal Function test (£99) includes TSH, FT3, FT4, and key hormonal markers — a focused investigation into the most common medical cause of unexplained weight gain.

For a broader metabolic assessment, the Core Health 45 test (£120) covers thyroid function, HbA1c, lipid panel, liver function, vitamin D, iron studies, and inflammation markers. This is the best option if you want to investigate multiple possible causes in one test.

Women who suspect hormonal weight gain — particularly those with irregular periods, acne, or perimenopausal symptoms — should consider the Female Hormones Clarity 31 test (£115), which includes cortisol, DHEA-S, full reproductive hormones, thyroid, and metabolic markers.

Unexplained weight gain has an explanation — it's just hidden in your blood. Stop blaming willpower and start investigating the actual cause. Order your test, get results in 2–3 working days, and take the first step toward understanding what's really going on.