How Your Thyroid Affects Weight

Anyone who has ever tried to lose weight knows the frustration. The endless dieting. The attempts to exercise. The ups and downs on the scale. When at long last your doctor figures out you have a thyroid problem, you pin your hopes on the thyroid hormone replacement drug, only to find, months later, that you're still embroiled in a battle with your weight.

Or perhaps you're part of the minority who has the opposite problem. No matter how much you eat, you can't seem to gain weight. You try eating more frequently. You try to eat more high-calorie foods. You indulge in desserts, snacks, and anything you want. Still, you remain thin as a rail and forever fighting off colds, flus, and other infections.

In both cases, the problem could have something to do with your thyroid. An underactive thyroid tells the body to slow down and harness its energy stores, causing an increase in appetite and an overall reduction in your metabolism. An overactive thyroid speeds everything up, so that cell activity is accelerated, causing energy in your body to be burned more quickly. (This chapter focuses on losing weight since that is the more common problem.)

Some people, like Beth, encounter weight changes on both ends of the spectrum.

Early on, Beth noticed she was losing weight, and had headaches, exhaustion, and a rapid heartbeat. A doctor uncovered her hyperthyroidism and suggested RAI treatment. Before the treatment, he encouraged her to eat whatever she wanted, and still, she lost 10 percent of her body weight. But after the RAI, her doctor waited six weeks before starting her on Synthroid. All the weight she lost came back until she got on Synthroid. Gradually, she lost the extra weight.

The thyroid gland plays a major role in body weight because it regulates your basal metabolic rate (BMR). That's the rate at which your body uses energy when it's at rest. In the old days, measuring your BMR was one way to determine whether you had a thyroid disease. Those who had a low BMR had an underactive thyroid. Those with a high BMR had an overactive thyroid.


Technically speaking, BMR is the rate of metabolism when you're at rest in a warm environment and have not eaten for at least twelve hours. The energy you require in this state is enough just for the vital organs such as the heart, lungs, nervous system, and kidneys. Factors that affect BMR include illness, stress, and the temperature of your environment. BMR declines with age and the loss of muscle mass.

Doctors no longer use BMR to determine your thyroid status, but a slowdown or speeding up in your metabolism is still a good indicator that something is awry with your thyroid. Getting a handle on your thyroid condition can usually put an end to this change in metabolism. But for some people, the weight problems do not go away with treatment.

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