Unexplained weight loss can be concerning and may stem from various underlying health issues. One potential cause is hyperthyroidism, a condition where the thyroid gland produces excessive hormones, leading to accelerated metabolism. 

The thyroid is a butterfly-shaped gland located at the base of your neck. It plays a crucial role in regulating your metabolism. It produces two hormones: Triiodothyronine and Thyroxine commonly called T3 and T4. 

These hormones help regulate basal metabolic rate, heart rate and cardiac output, body temperature, energy expenditure and metabolism of food. 

In a healthy individual, the hypothalamus in the brain releases Thyrotropin-releasing hormone, which signals the pituitary gland to produce thyroid-stimulating hormone.  thyroid-stimulating hormone instructs the thyroid to produce T3 and T4 at required levels. This feedback loop maintains metabolic homeostasis. 

Hyperthyroidism, or an overactive thyroid gland, occurs when the thyroid gland produces excessive amounts of thyroid hormones.  

The relationship between hyperthyroidism and weight loss is primarily attributed to an increased resting energy expenditure. Elevated thyroid hormone levels boost metabolic activity, causing the body to burn calories more rapidly, which can lead to weight loss even if food intake remains constant or increases. The study found that 85% of patients with untreated hyperthyroidism experience weight loss despite normal or increased food intake. 

More connection between weight loss and hyperthyroid: 

Recent research has shown that hyperthyroidism can increase BMR by 30-75% compared to normal thyroid function. This means your body burns significantly more calories at rest. 

Excess thyroid hormone activates hormone-sensitive lipase, an enzyme that breaks down stored triglycerides in adipose tissue. This results in increased free fatty acid release into circulation, greater fatty acid oxidation and reduced fat storage capacity. 

Thyroid hormones stimulate the sympathetic nervous system, accelerating gut motility and potentially leading to malabsorption. Prolonged hyperthyroidism can lead to muscle wasting. 

It is crucial to recognise that unexpected weight loss has numerous potential causes beyond hyperthyroidism. Medical conditions associated with unintentional weight loss include gastrointestinal disorders, endocrine disorders, infectious disorders, malignancies, psychological conditions and medication effects. 

Unexplained weight loss can be hyperthyroidism, due to the thyroid gland’s influence on metabolic rate. If you are experiencing unintended weight loss accompanied by symptoms like rapid heartbeat, anxiety or heat intolerance, it is essential to consult a healthcare professional for a thorough checkup. 


Kripa, is a Specialist Dietitian at The London Obesity & Endocrine Clinic. She has helped many patients overcome weight management barriers. ©Simplyweight