Hormones and Weight Gain: What Blood Tests to Ask For

You’re eating well, exercising regularly, and getting reasonable sleep — yet the scales refuse to budge, or worse, they’re creeping upward. If this scenario feels familiar, you’re far from alone. Unexplained weight gain is one of the most common frustrations Australians bring to their GPs, and in many cases, the underlying cause is hormonal.

Hormones are the body’s chemical messengers, regulating everything from metabolism and appetite to fat storage and energy expenditure. When even one hormone falls out of balance, it can create a cascade of metabolic effects that make weight management feel impossible. The key to breaking through this frustration lies in targeted blood testing that identifies exactly which hormones are contributing to the problem.

Thyroid Hormones: The Metabolic Master Switch

The thyroid gland produces hormones that directly control your basal metabolic rate — the number of calories your body burns at rest. When thyroid function is underactive (hypothyroidism), metabolism slows significantly, leading to weight gain, fatigue, cold intolerance, and difficulty losing weight even with caloric restriction.

The essential tests to request are TSH, free T3, and free T4. Many GPs will only order TSH as a screening tool, but this alone can miss subtle thyroid dysfunction. Free T3 is particularly important because it is the active thyroid hormone that drives cellular metabolism. A study published in the European Journal of Endocrinologyfound that even mildly elevated TSH levels within the "normal" reference range were associated with increased body weight and BMI (Knudsen et al., 2005).

Thyroid antibodies (TPO and TG antibodies) should also be considered if autoimmune thyroid disease is suspected, as Hashimoto’s thyroiditis is the most common cause of hypothyroidism in Australia and can cause fluctuating weight changes.

Insulin and Blood Sugar: The Fat Storage Connection

Insulin is arguably the most important hormone in weight regulation. Its primary role is to shuttle glucose from the bloodstream into cells for energy. However, when cells become resistant to insulin’s signal — a condition known as insulin resistance — the body compensates by producing more insulin. Elevated insulin levels promote fat storage, particularly around the abdomen, and make it extremely difficult to lose weight.

The most revealing tests are fasting insulin and HbA1c. Fasting glucose is commonly ordered but can remain normal for years while insulin resistance develops silently in the background. Research from the Journal of Clinical Investigationhas demonstrated that hyperinsulinaemia (excess insulin) is a key driver of obesity, independent of caloric intake (Ludwig & Ebbeling, 2018). A fasting insulin level above 10 mIU/L, even with normal glucose, may indicate early insulin resistance warranting dietary and lifestyle intervention.

Cortisol: The Stress Hormone

Chronic stress is an often-overlooked contributor to weight gain. When you’re under sustained stress, the adrenal glands produce elevated levels of cortisol. Chronically high cortisol promotes visceral fat accumulation (the dangerous fat surrounding internal organs), increases appetite — particularly for high-sugar and high-fat foods — and impairs sleep quality, creating a vicious cycle of weight gain.

A morning cortisol blood test can provide a useful snapshot, though salivary cortisol testing across the day (cortisol awakening response) offers a more comprehensive assessment of adrenal function. Cushing’s syndrome, while rare, should be excluded in cases of significant unexplained weight gain with cortisol elevation. Even subclinical cortisol elevations have been linked to increased abdominal obesity and metabolic syndrome (Anagnostis et al., 2009).

Sex Hormones: Oestrogen, Progesterone, and Testosterone

Hormonal shifts related to sex hormones affect weight in both men and women, though in different ways. In women, declining oestrogen levels during perimenopause and menopause are associated with increased abdominal fat deposition and reduced lean muscle mass. Progesterone deficiency can contribute to fluid retention and bloating, often perceived as weight gain.

For women, a hormonal panel should include oestradiol (E2), progesterone, FSH, LH, and SHBG. For men, the key tests are total and free testosterone, oestradiol, and SHBG. Low testosterone in men is strongly associated with increased body fat, reduced muscle mass, and metabolic syndrome. A comprehensive review in Obesity Reviewsconfirmed that testosterone deficiency is both a consequence and a cause of obesity in men, creating a self-perpetuating cycle (Kelly & Jones, 2015).

Leptin: The Satiety Signal

Leptin is a hormone produced by fat cells that signals to the brain when you’ve had enough to eat. In theory, people with more body fat should produce more leptin and therefore feel less hungry. In practice, many overweight individuals develop leptin resistance — their brains stop responding to the satiety signal, leading to persistent hunger and overeating despite adequate energy stores.

While leptin testing is not routinely offered in standard pathology panels, it can be requested through some Australian pathology providers. Elevated leptin levels in the context of obesity suggest resistance and can help guide treatment strategies focused on improving leptin sensitivity through dietary and lifestyle modifications.

Putting It All Together

If you’re struggling with unexplained weight gain, consider requesting the following blood tests from your GP or through a comprehensive health assessment:

• TSH, free T3, free T4, and thyroid antibodies
• Fasting insulin, fasting glucose, and HbA1c
• Morning cortisol
• Sex hormone panel (oestradiol, progesterone, testosterone, SHBG, FSH, LH)
• Leptin (if available)
• Full lipid panel and liver function tests (to assess metabolic syndrome)

At Yearly, we help you make sense of these results by analysing your blood work through our AI-powered platform. Upload your pathology PDF, and we’ll extract your biomarkers, identify hormonal imbalances, and provide actionable insights tailored to your specific situation. Because weight gain isn’t always about willpower — sometimes it’s about hormones, and the right blood test can prove it.

References

1. Knudsen, N., et al. (2005). "Small Differences in Thyroid Function May Be Important for Body Mass Index and the Occurrence of Obesity in the Population." European Journal of Endocrinology, 153(1), 39–44.
2. Ludwig, D.S. & Ebbeling, C.B. (2018). "The Carbohydrate-Insulin Model of Obesity: Beyond Calories In, Calories Out." JAMA Internal Medicine, 178(8), 1098–1103.
3. Anagnostis, P., et al. (2009). "Clinical Review: The Pathogenetic Role of Cortisol in the Metabolic Syndrome." Journal of Clinical Endocrinology & Metabolism, 94(8), 2692–2701.
4. Kelly, D.M. & Jones, T.H. (2015). "Testosterone and Obesity." Obesity Reviews, 16(7), 581–606.