Thyroid and Weight Loss or Weight Gain

Many people fight a long battle against being overweight, from early adulthood, or pregnancy, or even starting in childhood. Sometimes people are surprised to see a pattern of weight gain begin to level off, or even reverse, such that weight control or weight loss may occur without any obvious additional effort on their part. If unexplained weight loss occurs, especially with a good appetite, it might be because of overactive thyroid, also called hyperthyroidism [hie-per-THIGHroid- is-m] or thyrotoxicosis [thigh-ro-tox-i-KO-sis]. With hyperthyroidism, the body burns up extra food without using it for anything but for producing heat. People who may have been gaining weight before the onset of hyperthyroidism eventually begin to experience symptoms or problems that are less welcome than the loss of weight.

If your doctor diagnoses you with overactive thyroid, the first step is to learn whether it might be a temporary case, and whether it is mild. The next step is to talk with your doctor about what treatment is best. Sometimes it is best to wait it out to see whether the thyroid will return to normal on its own. The thyroid may even become underactive, after having been overactive.

However, untreated hyperthyroidism often will fail to resolve on its own. Thinning of the bones may occur if it is severe and untreated for a long time. Irregular heart rhythm, heart failure, or even death can result from a severely overactive thyroid. Sometimes people are treated with pills, especially for types of hyperthyroidism that are severe but possibly capable of future self-correction.

At other times, the treatment consists of swallowing a pill of radioactive [ray-dee-o-AK-tiv] iodine. However, radioactive iodine for overactive thyroid is one of the most common causes of underactive thyroid. Another option may be surgery.

For people who once had overactive thyroid (hyperthyroidism) and who also have been overweight, one of the most frustrating outcomes is the weight gain that may occur once the overactive thyroid has been treated. Weight gain after treatment of hyperthyroidism is related, in part, to whether there was already a tendency toward becoming overweight. It is also related to how much weight loss had occurred before treatment. Some people will entirely regain the amount of weight lost during hyperthyroidism after they are treated for overactive thyroid, and they might gain more than before the hyperthyroidism started.

When a person is recovering from hyperthyroidism, one of the special skills of the endocrinologist [en-doh-cri-NAlo-jist] is to know when to start the patient on treatment for underactive thyroid (hypothyroidism [hie-po-THIGHroid-is-m]). When the thyroid’s condition is changing rapidly, testing is interpreted differently than when thyroid status is stable. The risk of treating hypothyroidism too soon is that thyroid hormone replacement therapy could result in too much thyroid hormone. However, once it is known that hypothyroidism has occurred, then the patient usually requires lifelong treatment with thyroid hormone (levothyroxine [le-vo-thigh-ROX-een; [T4]). The risk of delaying treatment is that a person may gain more weight than otherwise might have occurred. Sometimes the amount of weight gain may approach or exceed 10 or 20 lbs.

What about other causes of hypothyroidism? There are temporary situations in which hypothyroidism may be mild and not require treatment. The most common cause of spontaneous permanent hypothyroidism is the gradual destruction of thyroid function by Hashimoto’s thyroiditis (from cells of the immune system that develop in the body that destroy the thyroid gland’s ability to function).

Weight gain from spontaneous, longstanding hypothyroidism may be very small compared to the weight gain sometimes seen after treatment of hyperthyroidism. Weight gain from spontaneous hypothyroidism may be 5-10 lbs. Weight gain in advanced severe hypothyroidism may contribute to obstructive sleep apnea (an inability to breathe leading to frequent awakening during sleep and daytime sleepiness). Some of the weight gain in severe cases of hypothyroidism is due to myxedema [mix-uh-DEE-muh] (excess fluid under the skin), which goes away during treatment.

Small differences in dose of thyroid hormone can make a big difference in whether your health will be the best it can be. A blood test called TSH (thyroid stimulating hormone) helps find the best thyroid dose. TSH reacts to blood levels of thyroid hormone like a thermostat. If thyroid levels are low, this test will show higher than normal levels of TSH. This test gives the right answer assuming the pituitary is working as it should. The amount of weight loss one can achieve having their severely underactive thyroid treated is modest at best.

If hypothyroidism was not present in the first place, then treatment with thyroid pills creates no advantage over allowing your thyroid to produce the needed amount of thyroid hormone.

Where does this leave the person who is being treated for underactive thyroid and still is having trouble achieving or maintaining ideal body weight, or the overweight person who is considering thyroid treatment but has been found to have normal thyroid function? Thyroid hormone should not be offered for weight loss if a person does not have a thyroid problem. Lifestyle changes may be needed to address unwanted weight gain or inability to lose weight. In other words, fewer calories and more physical activity – don’t cut corners!

Dr. Susan S. Braithwaite serves as Staff Physician at St. Francis Hospital, Evanston, IL, and Visiting Clinical Professor at University of Illinois—Chicago. She completed undergraduate studies at Harvard and Radcliffe in 1965 and medical school at the University of Chicago in 1969, where she went on to complete medical residency training and a fellowship in Endocrinology and Metabolism. Dr. Braithwaite is a member of the AACE Board of Directors.