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Understanding Weight Gain and Weight Loss Related to Hyperthyroidism

© Elaine Moore

People with hyperthyroidism often experience weight fluctuations, gaining or losing weight depending on the stage of their disorder.

Weight Gain or Loss with Active Hyperthyroid Disorder

During the active stage of a hyperthyroid disorder, when FT4 or FT3 levels are elevated, most people will lose weight without dieting. This type of weight loss is not considered healthy because muscle is frequently broken down in the process.

Hyperthyroidism is also associated with malabsorption, a condition in which food is metabolized too quickly for some nutrients to be absorbed properly. Many of the nutrient deficiencies related to hyperthyroidism can cause food cravings, muscle weakness, fatigue and other symptoms.

Most people with hyperthyroidism lose weight before their disorder is properly diagnosed and treated. Hyperthyroidism develops slowly, and it can be years before patients seek medical intervention. In the meantime, these patients can develop unhealthy eating habits and a feeling that they can eat whatever they want without gaining weight.

On the other hand, approximately 20% of hyperthyroid patients, especially younger ones, gain weight during the active phase of hyperthyroidism. The exact reasons for this are unclear.

Weight Gain or Loss after Treatment

Hyperthyroid patients who initially lose weight may find that this weight loss can't be sustained when their hyperthyroidism is corrected. On the recommended doses of anti-thyroid drugs, patients gain an average of five pounds within the first few weeks of treatment. When the dose of medication is reduced after the first 6-8 weeks, this weight gain usually stops.

However, patients who have developed bad eating habits or an increased appetite can continue to gain weight. It's not unusual for patients to gain back any weight loss during the height of their hyperthyroidism. Conversely, hyperthyroid patients who start out gaining weight are often able to lose weight when they start anti-thyroid drugs.

For either type of patient, treatment for hyperthyroidism that results in permanent hypothyroidism is likely to cause significant weight gain over time. Patients who have radioiodine ablation are more likely to gain weight as the severity of their hypothyroidism worsens over time.

Patients who have partial thyroidectomies may initially gain weight due to their lowered metabolism, but are often able to resume and maintain normal thyroid levels within 6 months after surgery. Patients who achieve remission with anti-thyroid drugs often can do this as well.

Thyroid Replacement Hormone and Weight Loss

The difficulty in finding an optimal dose of replacement hormone contributes to weight problems in aggressively treated patients. Doses of replacement hormone that cause high levels of either FT4 or FT3 will contribute to weight gain presumably because of the appetite associated with excess thyroid hormone.