Weight Gain: Blame the Thyroid Gland?

Small Changes in Thyroid Function Can Cause Big Changes

You train hard several days per week, jacking up your heart rate in spin class, running intervals on the treadmill, and burning your muscles with free weights. Your workouts, combined with a sound diet, should have resulted in a magazine-cover body by now. But you’ve actually gained “unexplained” fat in the last several months or years that has stubbornly glued itself to your body. Surely something is amiss— and the culprit may be an ever-so-slight change in your thyroid functioning, one that is routinely dismissed by physicians as “being within normal limits.” But what if these changes have resulted in weight gain over time? In this case, “normal” might not be so normal.

The thyroid is a small gland located in the front of the neck that uses iodine to make the hormones thyroxine (T4), triiodothyronine (T3) and calcitonin. Through the actions of these hormones, the thyroid gland plays important roles in regulating metabolism and calcium balance. When your physician is examining thyroid functioning, she may order the following tests: TSH, T3 and T4. Serum thyrotropin (TSH) is produced by the pituitary gland and signals the thyroid gland to make and release T3 and T4.

A TSH test measures the amount of thyroid stimulating hormone in your blood. Normal values are 0.4-4.0 mIU/L. However, a TSH of 2.0 mIU/L or over, with normal T4, indicates a mildly underactive thyroid or clinical hypothyroidism, and should be monitored closely by a physician. An even higher TSH, out of the range, is indicative of hypothyroidism. Low TSH may indicate hyperthyroidism or even secondary hypothyroidism.

Normal triiodothyronine (T3) values are 100-200 ng/dL. T3 levels are high with hyperthyroidism. Thyroxine (T4) normal values are 4.5-11.2 mcg/dL. High T4 combined with a low TSH may indicate hyperthyroidism, whereas low T4 may indicate hypothyroidism. However, these values are not crystal clear— high or low values for one test may be a sign of other problems and not just thyroid functioning. Therefore, your physician will look at your comprehensive medical history and may order other tests prior to making a firm diagnosis.

For years we’ve known that hypothyroidism, i.e., when the thyroid gland doesn’t produce enough thyroid hormone, causes weight gain— whereas hyperthyroidism, an overactive gland, typically leads to weight loss (and weight re-gain once treated). Recent research indicates that seemingly small changes within the normal range of thyroid activity may lead to weight gain. In the Framingham Offspring Study, researchers set out to examine if variations in thyroid functioning within the reference range are associated with bodyweight or changes in bodyweight over time. They examined fasting TSH and changes in bodyweight during the 3.5-year study of 2,407 men and women.

The findings from this study have shed new light on thyroid functioning. Initial TSH concentrations as measured at the start of the study were strongly correlated with bodyweight. The higher a participant’s TSH (remember, TSH is often high when thyroid functioning slows), the higher their weight. And increases in TSH levels over the course of the study were associated with increased bodyweight. By the end of the study period, even modest increases in TSH within the normal range of thyroid functioning were associated with weight gain. An increase in 1U of log TSH concentrations was associated with a 4.2 kg (1.9-pound) greater weight gain in women and 1.9 kg (0.86-pound) greater weight gain in men.

Do the results of this study mean that doctors should be prescribing medicine to boost thyroid functioning if your TSH level is within normal limits but has edged up slightly over time? While this may seem like a quick fix, it’s too early to play the thyroid blame game and start handing out medication to every person with a higher TSH and a few pounds of extra weight. After all, every medication comes with side effects and, at this time, we don’t know if boosting thyroid functioning through medication will reverse the weight gain.

In addition, though in this study, the researchers couldn’t account for the weight gain with changes in diet and physical activity, people typically underestimate their calorie intake while overestimating their calorie output through physical activity. Though scientists do the best they can to ensure that food intake and exercise output are measured accurately, these variables are self-reported in large studies like the Framingham Offspring Study. And, the reality is that most people underestimate how much they eat and exercise and the intensity level of their exercise. Not to mention the fact that you are doing yourself a disservice if you rely on the “calories burned” estimates on the machines at your gym, since these values are typically much higher than what you actually burned during exercise.

If you think your thyroid is to blame for your weight gain, what can you do? First, figure out if you have any of the symptoms of hypothyroidism including a lack of energy, depression, constipation, weight gain, hair loss, dry skin, dry and coarse hair, muscle cramps, decreased concentration, aches and pains, swelling of the legs, and increased sensitivity to cold. If your symptoms align with some of these, get your thyroid hormones tested. Then, talk to your physician about the test results (don’t settle for a phone call saying “your tests are normal”) and, get a copy for your records.

Even if your results are normal, it is important to have all of your test results in your own files so you can look at changes in your lab results over time and talk to your physician about any changes you think may be problematic. Though it is likely premature for physicians to prescribe thyroid hormone if your levels are still within the normal range, they may have other solutions like considering your iodine intake, especially if you consume minimal to no salt. The iodine in our diet today primarily comes from iodized salt since food naturally contains very little iodine. Avoid salt like the plague and you may find yourself with an iodine-deficient diet. And, the consequences of iodine deficiency are serious and can result in female infertility, autoimmune disease of the thyroid and an increased risk of thyroid cancer. A delicate balance is necessary; too much iodine is harmful for the thyroid.

Though thyroid functioning is one important determinant of bodyweight, it isn’t the sole factor, and thyroid medication may or may not be a quick fix, depending on how much weight you’ve gained. If you’re concerned about weight gain, never discount a cold, hard look at every single thing you are eating and drinking and conduct a reality check with your physical activity program. Keep track of both your diet and exercise with an online or paper journal and bring this information to your doctor. By doing so, she is more likely to take your concerns seriously.

Thyroid. National Center for Biotechnology Information. U.S. National Library of Medicine. U.S. Department of Health and Human Services. http://www.nlm.nih.gov/medlineplus/druginfo/natural/35.html