Thyroid cancer is a serious condition that can cause a variety of symptoms, but weight gain is not one of them. Patients who have undergone total thyroidectomy may experience some weight gain, but this is usually due to other factors such as hypothyroidism or medications. In general, the decrease in basal metabolic rate (BMR) due to hypothyroidism is much less dramatic than the marked increase seen in hyperthyroidism, leading to more modest alterations in weight. Weight gain is usually greater in people with more severe hypothyroidism, and is usually due to excessive salt and water accumulation rather than fat accumulation.
The most common sign of a thyroid disorder is an unexplained change in weight. If your thyroid produces more hormones than your body needs, you can lose weight unexpectedly. This is known as hyperthyroidism. Conversely, if your thyroid produces too few hormones, you can gain weight unexpectedly.
This is known as hypothyroidism. Unlike hyperthyroidism and hypothyroidism, thyroid cancer doesn't cause symptoms such as weight changes, heart palpitations, and thinning hair. The tumor can grow to invade nearby tissue and can spread (metastasize) to the lymph nodes in the neck. Sometimes, cancer cells can spread beyond the neck to the lungs, bones, and other parts of the body. Massive weight gain is rarely associated with hypothyroidism.
In general, 5-10 pounds of body weight can be attributed to the thyroid, depending on the severity of the hypothyroidism. If weight gain is the only symptom of hypothyroidism that is present, the weight gain is less likely to be due solely to the thyroid. Other symptoms include hoarseness, thyroid pain, and difficulty swallowing that doesn't go away for weeks or months. Common treatments for hyperthyroidism include radioactive iodine therapy and antithyroid medication. Radioactive iodine therapy destroys part or all of the thyroid gland and reduces hormone production.
Antithyroid medication aims to reduce the amount of hormones produced by the thyroid. Once hypothyroidism has been treated and thyroid hormone levels are in the normal range, the ability to gain or lose weight is the same as in people who don't have thyroid problems. Benign thyroid nodules can sometimes be left alone (not treated) and closely watched, as long as they don't grow or cause symptoms. Parathyroid gland cancers are very rare; there are probably fewer than 100 cases each year in the United States. If you experience unexplained weight changes or other symptoms of a thyroid disorder, it's important to talk to your doctor about your risk for thyroid cancer and your treatment options. Discussing your options with a genetic counselor can help you understand your risk for thyroid cancer and make an informed decision about your treatment.