Thyroid and Weight Gain: Is Your Thyroid to Blame?

An underactive thyroid (hypothyroidism) can cause weight gain, but usually a modest amount: the American Thyroid Association puts it at about 5 to 10 pounds, depending on severity, mostly from retained salt and water. If you've gained much more than that, other causes are likely involved. A TSH blood test is the usual first check.

How does the thyroid affect your weight?

Thyroid hormone helps set your metabolism, the rate at which your body turns food into energy. When the thyroid makes too little hormone, your body burns fewer calories and holds on to extra salt and water, which shows up on the scale.

The American Thyroid Association explains that most of the weight gained with hypothyroidism is salt and water, and that the change is usually much less dramatic than the weight loss seen with an overactive thyroid. More severe hypothyroidism can mean more weight gain.

What other symptoms point to an underactive thyroid?

Common symptoms include fatigue, feeling cold when others don't, joint and muscle pain, dry skin, dry or thinning hair, heavy or irregular periods, a slow heart rate and depression. Weight gain with several of these is a stronger reason to test than weight gain alone.

Those symptoms come from NIDDK, which notes that women are much more likely than men to develop hypothyroidism, and that it's more common after age 60. Causes include Hashimoto's disease, inflammation of the thyroid, thyroid surgery or radiation, and some medicines. Because the symptoms come on slowly and overlap with everyday tiredness and aging, they're easy to miss. Since some medicines can cause hypothyroidism, bring a list of everything you take, including supplements, when you ask about testing.

How is hypothyroidism tested?

The usual first test is a TSH (thyroid-stimulating hormone) blood test. A high TSH often means the thyroid isn't making enough hormone, because the pituitary gland is pushing it to work harder.

MedlinePlus explains that a TSH level that's too high or too low may be a sign of a thyroid problem. If the result isn't normal, your provider will probably order more tests, such as T4, T3 or thyroid antibodies, to find the cause. A TSH test alone doesn't require fasting, but you may need to fast if other blood tests are drawn at the same time.

Will treating hypothyroidism take the weight off?

Often not by much. Treatment mainly removes the extra water, and in one study of adults starting thyroid medicine, weight barely changed on average and only about half lost any weight.

That 2014 study of 101 adults with newly diagnosed hypothyroidism found a median weight loss of just 0.1 kilograms once thyroid levels were back in range, and only 52% lost weight. The American Thyroid Association adds that if your other thyroid symptoms improve with treatment but your weight doesn't, the weight gain probably wasn't caused only by your thyroid.

How is an underactive thyroid treated?

Hypothyroidism is treated with levothyroxine, a daily pill identical to the hormone a healthy thyroid makes. Your dose is checked with a blood test about 6 to 8 weeks after you start and adjusted if needed.

NIDDK describes this follow-up testing. MedlinePlus notes levothyroxine is usually taken once a day on an empty stomach, 30 minutes to 1 hour before breakfast, and that it controls hypothyroidism but doesn't cure it, so you keep taking it even when you feel well.

Can thyroid medicine be used for weight loss?

No. Levothyroxine carries a boxed warning that it should not be used, alone or with other treatments, to treat obesity or cause weight loss. Taking thyroid hormone you don't need can cause serious harm.

The warning is in the MedlinePlus drug information for levothyroxine. If your thyroid tests are normal and your weight is still climbing, the answer is to look for the other causes, not to add thyroid hormone.

What if your thyroid is normal but you're still gaining weight?

Then the weight has other causes, such as eating and activity patterns, sleep, stress, medications, other hormone changes or age. That's common, and it's treatable with the right plan.

A normal thyroid result is still useful: it rules out one cause and points the conversation toward the ones you can work on. Keep a simple record of your weight over the past year or two, when the gain started, and any changes in medications, sleep or routine around that time. Those details help your provider see the pattern quickly.

At Padgett Medical Center in Tampa, if you're coming in for a hormone or weight-loss visit, mention your symptoms and ask whether thyroid testing makes sense for you. Care is lab-guided and medically supervised, and Morgan Colee, PA, reviews your results with you. If your thyroid needs its own treatment, your provider will talk through the next step. For weight, see medical weight loss in Tampa; for hormones, see hormone replacement therapy. Visits are in person at 6904 W. Linebaugh Ave or by telehealth in Florida. Book your visit.

This article is educational and is not medical advice. Talk with a licensed provider about your own health, medications and treatment options.