Two people in my life have taught me that weight gain isn't always what it looks like from the outside.
The first is my dad. I wrote about him in the cholesterol article and the HbA1c piece. He gained about six kilograms over three years. Slowly. Invisibly. He didn't change his diet. He didn't stop exercising. His GP spotted it during a routine check, not because he'd complained, but because she was tracking his waist circumference alongside his blood work. His HbA1c had crept into pre-diabetic range. His metabolic environment was shifting, and the weight was following.
The second is my girlfriend. Her PCOS story is woven through several articles in this series: iron, PCOS, fertility, hair loss. But the weight dimension is the one she talks about least and feels most. She's been told, directly and indirectly, for most of her adult life that she just needs to try harder. Eat less. Move more. Have more discipline. And she's done all of those things, consistently, genuinely, for years. The weight barely shifts. Because the metabolic environment created by PCOS and insulin resistance can make weight regulation fundamentally harder. Not impossible. Harder. The inputs that produce results for other people produce smaller, slower, more frustrating results for her.
This is the article I've wanted to write since we started this series. Not because blood tests solve weight problems. They don't. But they can identify whether something metabolic or hormonal is making the problem harder than it should be. And that distinction matters. It matters clinically, because the treatment changes. It matters emotionally, because understanding why your body isn't responding the way you expect is the difference between self-blame and informed action.
Two-thirds of Australian adults are overweight or obese, and three-quarters of Australian men, according to the Australian Institute of Health and Welfare. This isn't a fringe issue. It's the second-leading risk factor for disease burden in the country. And for a significant proportion of those people, the cause isn't a simple calorie surplus. It's a metabolic picture that blood tests can help illuminate.
A note before we get into it
General information only. I'm not a doctor, an endocrinologist, or a dietitian. Weight management is complex, personal, and influenced by far more than blood results, including genetics, environment, mental health, socioeconomic factors, medication, sleep, and stress.
Blood tests can identify physiological contributors. They don't explain the full picture, and they don't provide a management plan. If you're struggling with weight and concerned about underlying causes, your GP is the right starting point.
This article is deliberately careful about language. Weight is a sensitive topic, and I have no interest in shaming anyone or reducing a complex issue to a lab report.
Why "eat less, move more" isn't always the full story
I want to be clear about something upfront: for most people, the fundamentals of energy balance do matter. If you consistently eat more than you burn, you'll gain weight. That's physics, and no blood test changes it.
But "eat less, move more" assumes the relationship between input and output is linear. That your body processes food, stores fat, and burns energy in a predictable, consistent way. For many people, especially those with metabolic dysfunction, it doesn't.
Insulin resistance, for example, can change how efficiently your body stores fat and how readily it releases fat for fuel. Thyroid dysfunction can alter your basal metabolic rate. Cortisol dysregulation can shift fat distribution toward abdominal storage. PCOS can create a hormonal environment where weight regulation is biologically different. These aren't excuses. They're mechanisms. And they're identifiable.
The point of testing isn't to find a reason not to change your lifestyle. It's to understand whether your physiology is working against you in ways that standard advice doesn't account for, and if so, whether those factors can be addressed to make the lifestyle changes actually work.
The blood tests that identify metabolic and hormonal contributors
Thyroid Function (TSH, Free T4, Free T3)
How it connects to weight
Your thyroid sets your metabolic rate. When it underperforms (hypothyroidism), your metabolism slows. Sometimes subtly, sometimes significantly. The weight gain from hypothyroidism is typically modest (3-5 kg), but it's resistant to standard dietary changes because the metabolic rate itself is suppressed.
The subtle version
Subclinical hypothyroidism, where TSH is mildly elevated but Free T4 remains in range, is common and can contribute to gradual weight gain and difficulty losing weight without being obvious on a standard screen. The relationship is bidirectional too: weight gain itself can push TSH upward, creating a feedback loop.
What to test
TSH, Free T4, Free T3. For more detail, see our thyroid function test guide.
Test it with Bloody Good:
Product: Thyroid Function Test (TFT)
HbA1c and Fasting Glucose
How it connects to weight
Blood sugar dysregulation and insulin resistance are among the most common metabolic contributors to weight gain and weight loss resistance. When your cells don't respond efficiently to insulin, your body produces more to compensate. Elevated insulin can promote fat storage (particularly abdominal) and may inhibit fat breakdown.
The progression
Normal glucose, then insulin resistance (glucose normal but insulin elevated), then pre-diabetes (glucose starting to rise), then type 2 diabetes. At each stage, weight management tends to become harder.
What to test
HbA1c, fasting glucose. For more detail, see our HbA1c guide.
Fasting Insulin
Why this one matters
This is arguably the most underused test in the weight investigation toolkit. Fasting glucose and HbA1c can look normal while fasting insulin is already elevated. That means your body is producing excess insulin to maintain blood sugar control. That excess insulin is driving fat storage.
Early detection
Fasting insulin can detect insulin resistance earlier than glucose or HbA1c. If you're gaining weight despite good habits, and your glucose markers are fine, fasting insulin may reveal what glucose testing misses.
The HOMA-IR calculation
Some clinicians use fasting insulin and fasting glucose together to calculate HOMA-IR, a measure of insulin resistance. Values above 1.9 suggest early insulin resistance. Above 2.9 suggests significant insulin resistance.
Test it with Bloody Good:
Product: Fasting Insulin Blood Test
Cholesterol and Triglycerides
Not just a heart test
The lipid panel isn't usually thought of as a "weight test," but the pattern it reveals is clinically important. Elevated triglycerides plus low HDL is one of the hallmarks of metabolic syndrome, a cluster of risk factors that includes abdominal obesity, insulin resistance, and cardiovascular risk.
What the pattern tells you
High triglycerides often signal excess carbohydrate and alcohol intake, insulin resistance, or both. They tend to respond well to dietary changes, especially reducing refined carbohydrates and alcohol, which makes them a useful feedback marker.
For more detail, see our cholesterol guide.
Test it with Bloody Good:
Product: Cholesterol (Lipid Studies inc. HDL)
Cortisol
The stress-weight link
Cortisol, your primary stress hormone, directly influences fat storage, particularly in the abdominal area. Chronic stress can elevate cortisol, which may promote visceral fat deposition, increase appetite, and disrupt blood sugar regulation.
Limitations of a single test
A single morning cortisol blood test has limitations. Cortisol fluctuates throughout the day, and one reading may not capture chronic dysregulation. But it can identify clearly elevated or suppressed levels that warrant further investigation.
When it's worth checking
If your weight gain is concentrated in the abdomen, you're chronically stressed, your sleep is poor, and your other metabolic markers are borderline, cortisol may be a contributing factor worth exploring.
Test it with Bloody Good:
Product: Cortisol Blood Test
Testosterone and SHBG
For men
Low testosterone is associated with increased body fat, reduced muscle mass, and metabolic syndrome. Obesity itself can suppress testosterone, creating another feedback loop. Weight gain lowers testosterone, and low testosterone can make it harder to lose weight.
For women
In PCOS, elevated androgens and low SHBG (often driven by insulin resistance) can create a hormonal environment that favours weight gain, particularly around the abdomen. Addressing insulin resistance often improves both the hormonal picture and weight management.
What to test
Total and free testosterone, SHBG. For men, see our testosterone guide. For women with PCOS, see the PCOS guide.
Test it with Bloody Good:
Product: Testosterone Free/Total + SHBG
Liver Function (ALT/AST)
A metabolic signal, not just a liver test
Non-alcoholic fatty liver disease (NAFLD) is strongly associated with insulin resistance, metabolic syndrome, and obesity. It's one of the most common liver conditions in Australia and is often completely asymptomatic. Mildly elevated liver enzymes, particularly ALT, can be an early signal of metabolic liver stress.
The bigger picture
NAFLD isn't just a liver problem. It's a metabolic marker. Elevated ALT in the context of weight gain, elevated triglycerides, and borderline blood sugar markers paints a picture of systemic metabolic dysfunction.
Test it with Bloody Good:
Product: Liver Function Test (LFT)
Vitamin D
Weight and vitamin D
Obesity is associated with lower circulating vitamin D levels, partly because vitamin D is fat-soluble and gets sequestered in body fat. Whether low vitamin D contributes to weight gain or is a consequence of it is still debated. But correcting deficiency is part of optimising the metabolic environment.
Test it with Bloody Good:
Product: Vitamin D (25-OH) Blood Test
The metabolic pattern most people miss
The individual tests above are useful. But the real power is in the pattern.
Metabolic syndrome is defined by the clustering of three or more of the following: abdominal obesity (waist circumference above 94 cm in men, 80 cm in women, the Australian thresholds), elevated triglycerides, reduced HDL, elevated blood pressure, and elevated fasting glucose.
When these markers cluster, they indicate systemic insulin resistance. A metabolic state that can make weight gain easier, weight loss harder, and cardiovascular risk significantly higher.
The frustrating thing about metabolic syndrome is that no single marker looks dramatic on its own. Your triglycerides are a bit high. Your HDL is a bit low. Your fasting glucose is borderline. Your waist has crept up a few centimetres. Each number, individually, might not even be flagged on a standard lab report. But together, they form a pattern that your GP should recognise. One that changes the conversation about what's driving your weight.
My dad's story was exactly this. No single marker screamed crisis. But the GP who looked at the pattern (LDL elevated, triglycerides creeping, HbA1c 43, waist circumference up 4 cm over three years) saw what individual numbers in isolation would have missed.
When blood tests come back normal, and it IS lifestyle
I owe it to you to be honest about this: for many people who struggle with weight, blood tests will come back normal. No thyroid issue. No insulin resistance. No hormonal imbalance. No metabolic syndrome.
That's actually useful information, even though it's disappointing. It means the most common physiological contributors have been ruled out, and the focus can shift squarely to lifestyle factors with confidence that your body's machinery is working as expected.
The lifestyle factors that matter most for weight are, predictably, not glamorous. Sleep quality and duration (poor sleep disrupts hunger hormones and insulin sensitivity). Stress (chronic cortisol elevation). Alcohol (liquid calories plus metabolic disruption). Dietary quality, not just quantity, because food quality affects satiety, insulin response, and gut health. Physical activity (resistance training in particular for metabolic rate). And the food environment: what's available, affordable, and convenient shapes what we eat.
Normal blood results don't mean you've failed. They mean the solution is in a different domain, and it's worth investigating that domain with the same rigour you brought to the blood work.
Who should be testing
Anyone with weight gain that doesn't match their lifestyle. If you're genuinely active, eating reasonably, and still gaining, something may be off metabolically.
Anyone with weight loss resistance. Making changes, being consistent, and seeing no results (or results that plateau quickly) is worth investigating.
Women with PCOS or suspected PCOS. Insulin resistance is present in roughly 85% of PCOS cases and is a primary driver of weight management difficulty.
Men over 40 with increasing abdominal weight. The testosterone-obesity feedback loop and metabolic syndrome become increasingly common from the 40s onwards.
Anyone with a family history of type 2 diabetes or cardiovascular disease. Metabolic risk often clusters in families.
Anyone who's never had metabolic screening. If you're over 35, carry weight around your middle, and have never had fasting insulin, HbA1c, or lipids checked, this is a reasonable baseline.
How to prepare
Fast for 10-12 hours. Fasting insulin, fasting glucose, and lipids all require a clean fast. Water is fine.
Test in the morning. Insulin and cortisol follow circadian rhythms. Morning testing provides the most consistent results.
Avoid heavy exercise the day before. Intense training can affect glucose regulation and liver enzymes.
Continue normal medications unless your clinician advises otherwise.
Know your waist circumference. Measure at the level of the navel, standing, after a normal breath out. This number, combined with your blood results, helps your GP assess metabolic syndrome.
Understanding your results as a pattern
Look for the metabolic cluster, not individual flags.
| Pattern | What it suggests |
|---|---|
| High triglycerides + low HDL + borderline glucose | Metabolic syndrome / insulin resistance |
| Elevated TSH + weight gain + fatigue | Thyroid contributing to metabolic slowdown |
| Normal glucose + elevated fasting insulin | Early insulin resistance (glucose tests miss this) |
| Elevated ALT + high triglycerides + abdominal weight | NAFLD / metabolic liver stress |
| Low testosterone + abdominal weight gain (men) | Testosterone-obesity feedback loop |
| Elevated androgens + low SHBG (women) | PCOS-related metabolic dysfunction |
| All normal | Lifestyle factors are primary. Optimise sleep, stress, nutrition, movement. |
Tests to consider through Bloody Good
Core weight investigation panel
Thyroid Function Test (TFT) — TSH, Free T4, Free T3
HbA1c — 3-month blood sugar average
Fasting Glucose — blood sugar snapshot
Fasting Insulin — early insulin resistance detection
Cholesterol (Lipid Studies inc. HDL) — full lipid panel including triglycerides
Extended metabolic context
Liver Function Test (LFT) — NAFLD screening
Cortisol Blood Test — stress hormone
Testosterone Free/Total + SHBG — hormonal context (men and women)
Vitamin D (25-OH) — commonly low alongside metabolic dysfunction
If you want broad coverage
The Bloody Good Test covers 100 biomarkers including all the metabolic markers above plus FBC, iron studies, kidney function, inflammation, and more. For a full metabolic baseline, it covers everything in one blood draw.
What to do after testing
If a metabolic pattern is identified: Take the results to your GP. Metabolic syndrome, insulin resistance, and thyroid dysfunction all have established management pathways, typically starting with lifestyle intervention (nutrition, activity, sleep, stress) and sometimes medication depending on severity.
If insulin resistance is present: Dietary changes targeting blood sugar regulation (reducing refined carbohydrates, increasing fibre, balanced meals with protein and healthy fat) alongside regular physical activity, especially resistance training, are first-line. Your GP may consider metformin depending on the severity and your risk profile.
If thyroid dysfunction is confirmed: Treatment (typically levothyroxine for hypothyroidism) may help address the metabolic slowdown. Weight management often becomes more responsive to lifestyle changes once thyroid function is optimised.
If results are normal: Shift focus to the lifestyle fundamentals. Consider working with a dietitian who can help you identify what's actually going on with your nutrition. Not what you think is happening, but what the data shows. Sometimes the gap between perceived intake and actual intake is larger than people expect. Sleep, stress, and alcohol are worth interrogating with the same honesty.
Retest at 3-6 months if you're making changes. Fasting insulin, HbA1c, triglycerides, and liver enzymes are all responsive markers. They shift when the underlying drivers change.
Explore more biomarkers
Browse the Bloody Good Biomarker Directory
General information only. This article is not medical advice and is not a substitute for care from a qualified health professional. If you have concerning symptoms or urgent health issues, seek medical attention promptly.