Why can't I lose weight? Metabolism, biomarkers and what to test

Thyroid dysfunction and metabolic rate

The thyroid is the primary regulator of resting metabolic rate. When the thyroid is underactive (hypothyroidism), the body's calorie-burning processes slow down, making weight loss significantly harder even when energy intake is carefully controlled. Even modest increases in TSH within the standard reference range have been associated with reduced metabolic rate and weight gain in research published in peer-reviewed endocrinology literature. Standard NHS testing measures TSH alone, which misses the picture when the issue is impaired conversion of T4 (the storage form) to T3 (the active form) rather than insufficient T4 production. A comprehensive thyroid screen including Free T3 alongside TSH and Free T4 captures this conversion bottleneck, which is one of the most commonly overlooked contributors to weight loss resistance.

Insulin resistance: the most common metabolic block

Insulin resistance is the most frequently identified metabolic barrier to fat loss in clinical practice. When cells become less responsive to insulin, the pancreas compensates by producing more insulin to move glucose into cells. Elevated insulin actively inhibits fat breakdown (lipolysis) and promotes fat storage, particularly around the abdomen. This creates a physiological state in which the body is biochemically oriented toward fat retention regardless of calorie intake. In the UK, an estimated 6.3 million people are living with pre-diabetes, and many more are insulin resistant without knowing it. HbA1c (reflecting average blood glucose over three months) and fasting glucose together provide a picture of blood sugar regulation, but neither is as sensitive for early insulin resistance as they could be. Looking at these markers alongside the full metabolic panel gives a much clearer picture.

Cortisol and belly fat accumulation

Cortisol, the body's primary stress hormone, has direct effects on fat distribution. Chronically elevated cortisol promotes the accumulation of visceral fat around the abdomen by activating fat storage mechanisms in the liver and abdominal tissue specifically. It also increases appetite, drives cravings for high-calorie foods, and impairs thyroid hormone conversion through a mechanism shared with the previous section. People who eat carefully and exercise consistently but cannot shift abdominal weight frequently have chronic cortisol elevation as a contributing factor, whether from work stress, disrupted sleep, over-training, or a combination of these.

Hormonal imbalances in women

Oestrogen plays a key role in fat distribution and insulin sensitivity. As oestrogen declines during perimenopause and menopause, fat distribution shifts toward the abdomen, metabolic rate decreases, and insulin sensitivity tends to worsen. Women with polycystic ovary syndrome (PCOS), which affects around 1 in 10 women in the UK, commonly experience significant weight management difficulty driven by elevated androgens and insulin resistance working together. Testosterone in women influences muscle mass and metabolic rate; when levels are low, lean mass declines and fat gain becomes easier. SHBG (sex hormone binding globulin) levels determine how much testosterone is biologically active, providing additional context when standard testosterone measurements appear normal.

Low testosterone in men

In men, low testosterone is one of the most underdiagnosed contributors to weight management difficulty. Testosterone supports lean muscle mass, which drives resting metabolic rate. When testosterone is low, muscle mass declines, fat accumulates (particularly viscerally), and motivation for physical activity decreases. The relationship is circular: excess body fat converts testosterone to oestrogen via an enzyme called aromatase, further lowering available testosterone and worsening the metabolic picture.

Nutritional deficiencies that impair metabolism

Vitamin D deficiency is around 35% more common in people with obesity than in those with healthy body weight, reflecting a bidirectional relationship between fat tissue (which sequesters vitamin D) and reduced sun exposure. Low vitamin D impairs insulin sensitivity, muscle function, and thyroid hormone metabolism. Iron deficiency reduces exercise capacity and impairs aerobic energy production, making physical activity harder and recovery slower. B12 deficiency contributes to fatigue and reduced motivation for activity. These deficiencies are addressable once identified, and identifying them before starting a weight management programme means interventions work with the biology rather than against it.

A GP visit for weight management difficulties typically results in a TSH measurement and possibly an HbA1c, which misses the majority of the metabolic contributors described above. A weight loss blood test designed to capture the full metabolic picture covers the thyroid axis completely, assesses blood glucose regulation, includes relevant hormones, measures inflammatory status, and checks key nutrients.

TSH is the pituitary signal to the thyroid. Elevated TSH is usually the first sign that the thyroid is underperforming and metabolism is slowing. However, TSH can appear within range while metabolic dysfunction is still present at the conversion level.

Free T4 measures the storage form of thyroid hormone. Combined with Free T3, it reveals whether the conversion process is functioning effectively.

Free T3 is the active, usable form of thyroid hormone. This is the marker most directly linked to metabolic rate. Low Free T3 with normal TSH indicates conversion impairment, a pattern that causes genuine metabolic slowing but is rarely identified by standard GP testing.

HbA1c reflects average blood glucose over the preceding two to three months. Elevated levels indicate insulin resistance or pre-diabetes, both of which directly impair fat loss and drive ongoing weight gain.

Fasting glucose provides a snapshot of current blood sugar regulation. Best interpreted alongside HbA1c for a complete picture.

Cortisol measured in the morning establishes a baseline for stress hormone output. Elevated or dysregulated cortisol is a direct contributor to visceral fat accumulation and resistance to fat loss.

LDL and lipid panel provides metabolic context. Elevated triglycerides alongside low HDL is one of the clearest indicators of insulin resistance and metabolic syndrome.

Vitamin D deficiency is both a cause and a consequence of metabolic dysfunction. Correcting it improves insulin sensitivity and supports thyroid hormone metabolism.

Ferritin (iron stores) determines exercise capacity and aerobic energy production. Low ferritin makes sustained physical activity harder and recovery slower, directly limiting the exercise component of any weight management plan.

Homocysteine provides information about B vitamin metabolism and systemic inflammation, both of which interact with metabolic efficiency.

Optimising blood sugar regulation through diet

The dietary intervention with the most consistent evidence for improving insulin sensitivity and supporting fat loss is a whole-food dietary pattern that reduces rapid glucose spikes. This means prioritising complex carbohydrates with fibre (legumes, vegetables, oats), adequate protein (1.2-1.6g per kilogram of body weight), and healthy fats from fish, olive oil, nuts, and seeds. Reducing refined carbohydrates and added sugars reduces the insulin response at each meal, allowing the body to access stored fat as fuel between meals. The practical principle is consistent meal composition rather than calorie restriction as the primary tool. Tracking HbA1c and fasting glucose at 3-month intervals provides an objective measure of whether dietary changes are translating into improved insulin sensitivity.

Resistance training for metabolic rate

Lean muscle mass is the primary driver of resting metabolic rate. Resistance training builds and preserves lean mass, which increases the number of calories burned at rest regardless of activity level. Two to three sessions of progressive resistance training per week is supported by consistent evidence for improving insulin sensitivity, reducing visceral fat, and improving testosterone in both men and women. The effect on thyroid hormone conversion is also positive: regular exercise improves T4-to-T3 conversion through mechanisms including improved liver function and reduced inflammatory burden. Retesting Free T3 alongside ferritin and vitamin D after a consistent training period of 12 weeks or more quantifies the metabolic response to this specific change.

Sleep and cortisol management

Poor sleep is one of the most powerful drivers of cortisol elevation, insulin resistance, and appetite dysregulation. Research consistently shows that sleep restriction of even two to three nights increases cortisol levels, reduces insulin sensitivity, and elevates hunger hormones in ways that directly undermine fat loss efforts. Prioritising seven to nine hours of consistent sleep per night is not a peripheral lifestyle suggestion but a central metabolic intervention. Morning cortisol testing before and after a structured sleep improvement period provides objective evidence of whether the HPA axis is responding to the change.