Biology

Thyroidism is the condition arising from abnormal functioning of the thyroid gland, the butterfly-shaped endocrine gland in the neck. The gland secretes thyroxine (T4) and triiodothyronine (T3), which need iodine for their synthesis, and its output is controlled by TSH from the pituitary. Too little hormone is hypothyroidism; too much is hyperthyroidism.

Why imbalance changes body weight

Thyroid hormones set the basal metabolic rate (BMR), the rate at which cells burn glucose and fat to release energy. They also affect appetite, heat production and the breakdown of stored fat and glycogen.

• Hypothyroidism (too little hormone): BMR falls, so cells use less energy. Calories not burnt are stored as fat, and the person gains weight, feels tired and cold, and may develop a goitre if iodine is lacking.
• Hyperthyroidism (too much hormone): BMR rises, so food is oxidised quickly and stored fat is mobilised. The person loses weight despite a good appetite, and has a fast heartbeat, sweating and heat intolerance.

A balanced level of thyroid hormones therefore keeps energy intake and expenditure matched and the body weight steady.