Why am I losing weight with hyperthyroidism even though I eat more?

Extra thyroid hormone raises your basal metabolic rate, so your body burns more calories than usual. The ATA says the amount of weight loss depends on how overactive the thyroid is.

Will I gain weight back after hyperthyroidism treatment?

The ATA says weight lost because of extra thyroid hormone is reversed when thyroid levels return to normal. With antithyroid medicine, NIDDK notes that levels can take several weeks or months to reach the normal range, so expect the regain to follow that timeline.

Why am I gaining weight with hyperthyroidism?

Hyperthyroidism raises appetite, and the ATA notes that some people gain weight depending on how much more they eat. After treatment, the habit of eating more can continue. NIDDK also notes that almost everyone who has radioiodine therapy later develops an underactive thyroid, which may cause some weight gain. Ask your doctor to check your levels.

What should I eat to gain weight with hyperthyroidism?

Smaller meals more often, with calorie- and protein-rich foods such as whole milk, yogurt, cheese, eggs, fish, nut butters, nuts and olive oil, while avoiding large amounts of iodine from seaweed and supplements.

Should I avoid iodine if I have Graves' disease?

NIDDK says people with Graves' disease may be sensitive to harmful side effects from iodine, and that foods with large amounts of iodine, such as kelp, dulse and other seaweed, or iodine supplements may cause or worsen hyperthyroidism. Ask your team which foods and products to limit.

How to Gain Weight With Hyperthyroidism

By Mustafa Bilgic · Updated September 2026 · Based on guidance from NIDDK, the American Thyroid Association, MedlinePlus, the NHS, NIH and USDA FoodData Central.

NIDDK lists weight loss despite an increased appetite among the symptoms of hyperthyroidism, also called an overactive thyroid. The American Thyroid Association (ATA) explains why: high thyroid hormone levels raise the body's basal metabolic rate, so it burns more calories. It also says that weight lost because of extra thyroid hormone is reversed when thyroid levels return to normal. That makes treatment, not food alone, the main route back to your usual weight. Here is what to eat in the meantime, what to limit, and how to track your way back.

Educational, not medical advice. Hyperthyroidism needs a diagnosis and treatment from a doctor. Untreated, it can cause serious problems with the heart, bones and muscles. Do not start iodine or kelp supplements, and do not change or stop thyroid medicines on your own.

Key takeaways

Treatment that brings thyroid levels back to normal is what reverses the weight loss.
Until then, eat smaller meals more often and add calorie- and protein-rich foods.
Avoid large amounts of iodine from kelp, dulse, other seaweed and iodine supplements unless your doctor says otherwise.
Once treatment works, watch for overshoot: the bigger appetite habit can outlast the illness.

Why an overactive thyroid causes weight loss

NIDDK explains that thyroid hormones control the way the body uses energy, so with too much thyroid hormone, many of the body's functions speed up. The ATA adds that high thyroid hormone levels raise the basal metabolic rate, the energy the body uses at rest, so people with hyperthyroidism may burn more calories than usual and lose some weight. How much depends on how overactive the thyroid is.

NIDDK lists other symptoms that often come with the weight loss: a rapid or irregular heartbeat, nervousness, trouble sleeping, shaky hands, muscle weakness, trouble tolerating heat and frequent bowel movements. Older adults may instead lose their appetite or withdraw from people. The most common cause is Graves' disease, an autoimmune disorder in which the immune system attacks the thyroid and makes it produce too much hormone; others include overactive nodules, thyroiditis and too much iodine.

Treatment comes first

NIDDK says doctors treat hyperthyroidism to bring thyroid hormone levels back to normal, which prevents long-term health problems and relieves uncomfortable symptoms. The usual options are medicines, radioiodine therapy or thyroid surgery. Methimazole is the antithyroid medicine used most often, and beta blockers can reduce symptoms such as tremors, rapid heartbeat and nervousness until other treatments start working.

Treatment takes time to show on the scale. NIDDK notes that with antithyroid medicine it may take several weeks or months for thyroid hormone levels to move into the normal range. Because the ATA ties the weight loss to the extra hormone, expect the regain to follow your thyroid levels more than your plate.

Weight regain tracker

Enter your usual weight before hyperthyroidism, the lowest weight you reached, and today's weight. Weeks since treatment and height are optional; they add a weekly pace and a BMI check.

Where you are

Weight lost

Regained so far

Left to reach usual weight

Arithmetic from the numbers you enter, not a diagnosis. For a fair comparison, weigh yourself on the same scale at the same time of day.

After treatment: why weight can overshoot

The ATA says weight gain can even occur in people who did not lose weight, because they may have gotten used to eating more from the increased appetite during hyperthyroidism. So the target is your usual weight, not as much as possible. If the tracker shows you past your old weight, ease portions back toward your old habits.

Treatment can also swing the thyroid the other way. NIDDK notes that almost everyone who gets radioiodine therapy later develops hypothyroidism, which is treated with a daily thyroid hormone medicine, and the ATA explains that an underactive thyroid may cause some weight gain. If your weight keeps climbing, ask your doctor to check your thyroid levels. Our guide to hypothyroidism and weight gain covers that side.

The ATA also notes that because hyperthyroidism raises appetite, some people actually gain weight while it is active, depending on how much more they eat.

What to eat while your thyroid levels are high

NIDDK's diet advice for hyperthyroidism centers on iodine, covered below. For the calories, general advice applies: the NHS suggests adults who want to gain weight add around 300 to 500 extra calories a day, gradually. Its tips:

Eat smaller meals more often, with healthy snacks between meals.
Add extra calories to meals with cheese, nuts and seeds.
Have high-calorie drinks, such as milkshakes, between meals, and do not fill up on drinks before meals.
Add protein with beans, pulses, fish, eggs and lean meat.
Do not rely on chocolate, cakes and sugary drinks to gain weight.

The Dietary Guidelines for Americans, 2025 to 2030 set a protein goal of 1.2 to 1.6 grams per kilogram a day and advise full-fat dairy with no added sugars. The foods below bring calories, protein and, in many cases, calcium for your bones.

Food	Portion	Calories	Protein	Calcium
Sardines, canned in oil, with bones	1 can (3.75 oz)	191	22.6 g	351 mg
Peanut butter, smooth	2 tbsp	191	7.1 g	16 mg
Ricotta, whole milk	1/2 cup	186	9.3 g	255 mg
Scrambled eggs	2 large	182	12.2 g	81 mg
Greek yogurt, plain, whole milk	6 oz	165	15.3 g	170 mg
Almonds	1 oz	164	6.0 g	76 mg
Avocado	1/2 fruit	161	2.0 g	12 mg
Whole milk	1 cup	149	7.7 g	276 mg
Olive oil	1 tbsp	119	0.0 g	0 mg
Canned pink salmon, with bones	3 oz	117	19.6 g	241 mg
Cheddar cheese	1 oz	114	6.5 g	201 mg
Tofu, firm, calcium-set	1/2 cup	98	11.4 g	253 mg

Source: USDA FoodData Central, SR Legacy. Canned salmon and sardines are listed with their soft bones, which is where much of their calcium comes from.

More choices: high-calorie foods to gain weight. A standard calorie calculator may underestimate your needs while your thyroid is overactive, because extra thyroid hormone raises your basal metabolic rate.

Iodine: what to limit

Your thyroid uses iodine to make thyroid hormones. NIDDK warns that if you have Graves' disease or another autoimmune thyroid disorder, you may be sensitive to harmful side effects from iodine. Eating foods that have large amounts of iodine, such as kelp, dulse or other kinds of seaweed, may cause or worsen hyperthyroidism, and taking iodine supplements can have the same effect. NIDDK suggests asking your health care team:

which foods to limit or avoid,
about any iodine supplements you take, and
about any cough syrups or multivitamins you take, because they may contain iodine.

NIDDK notes that some medicines, including some heart medicines, may contain a lot of iodine. The NIH Office of Dietary Supplements lists seafood as generally rich in iodine, and dairy products and eggs as good sources. If your team puts you on a low-iodine diet before radioiodine treatment, follow their food list rather than the table above.

Protect your bones while you regain

NIDDK and MedlinePlus list thinning bones and osteoporosis among the problems untreated hyperthyroidism can cause. The NIH Office of Dietary Supplements (ODS) describes calcium as a mineral your body needs to build and maintain strong bones, and notes that vitamin D helps your body absorb calcium.

Age and sex	Calcium a day	Vitamin D a day
Adults 19 to 50	1,000 mg	15 mcg (600 IU)
Men 51 to 70	1,000 mg	15 mcg (600 IU)
Women 51 to 70	1,200 mg	15 mcg (600 IU)
Adults 71 and older	1,200 mg	20 mcg (800 IU)

Source: NIH Office of Dietary Supplements, calcium and vitamin D fact sheets for consumers.

ODS says milk, yogurt and cheese are the main food sources of calcium for most people in the United States, and canned sardines and salmon with bones contain calcium. Almost all U.S. milk is fortified with about 3 mcg (120 IU) of vitamin D per cup, and fatty fish such as salmon, trout, tuna and mackerel are among the best natural sources. Ask your doctor whether you need a supplement, and what exercise is safe while your heart rate is still high.

When to call your doctor

The NHS advises speaking to a GP if you have lost weight suddenly or without changing your diet or exercise routine. If you already have a diagnosis, tell your doctor if you keep losing weight after starting treatment. The NHS also lists a thyroid storm, a sudden and life-threatening flare-up of symptoms, among the problems when an overactive thyroid is not treated or well controlled. The ATA says it is very important to discuss any side effects of antithyroid medicine with your clinician, and NIDDK advises getting the disease under control before pregnancy.

Educational, not medical advice. This guide summarizes public guidance and does not replace care from your doctor or endocrinologist. Do not take iodine or seaweed supplements, and do not change thyroid medicines, without your doctor's advice. Unexplained weight loss should always be checked by a doctor. Operator: Mustafa Bilgic.

Frequently asked questions

Why am I losing weight with hyperthyroidism even though I eat more?: Extra thyroid hormone raises your basal metabolic rate, so your body burns more calories than usual. The ATA says the amount of weight loss depends on how overactive the thyroid is.
Will I gain weight back after hyperthyroidism treatment?: The ATA says weight lost because of extra thyroid hormone is reversed when thyroid levels return to normal. With antithyroid medicine, NIDDK notes that levels can take several weeks or months to reach the normal range, so expect the regain to follow that timeline.
Why am I gaining weight with hyperthyroidism?: Hyperthyroidism raises appetite, and the ATA notes that some people gain weight depending on how much more they eat. After treatment, the habit of eating more can continue. NIDDK also notes that almost everyone who has radioiodine therapy later develops an underactive thyroid, which may cause some weight gain. Ask your doctor to check your levels.
What should I eat to gain weight with hyperthyroidism?: Smaller meals more often, with calorie- and protein-rich foods such as whole milk, yogurt, cheese, eggs, fish, nut butters, nuts and olive oil, while avoiding large amounts of iodine from seaweed and supplements.
Should I avoid iodine if I have Graves' disease?: NIDDK says people with Graves' disease may be sensitive to harmful side effects from iodine, and that foods with large amounts of iodine, such as kelp, dulse and other seaweed, or iodine supplements may cause or worsen hyperthyroidism. Ask your team which foods and products to limit.

Keep reading

References

Sources: NIDDK: Hyperthyroidism (Overactive Thyroid) · American Thyroid Association: Thyroid and Weight · American Thyroid Association: Hyperthyroidism · MedlinePlus: Hyperthyroidism · NHS: Overactive thyroid · NHS: Healthy ways to gain weight · NIH ODS: Calcium · NIH ODS: Vitamin D · NIH ODS: Iodine · CDC: Adult BMI Categories · Dietary Guidelines for Americans, 2025 to 2030 · USDA FoodData Central.