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Weight gain is a common symptom of hypothyroidism (underactive thyroid gland or decreased thyroid function). Treatment of this thyroid disease with levothyroxine sodium (thyroid hormone replacement therapy) helps to restore normal thyroid hormone levels in the body. 

Taking levothyroxine to treat hypothyroidism can result in some minor weight loss after several months of treatment, but it does not cause clinically significant weight loss in most people. Please continue reading to learn more.

Do people with hypothyroidism lose weight on levothyroxine?

According to the American Thyroid Association, people with hypothyroidism (decreased thyroid function) do not lose a significant amount of weight on levothyroxine sodium. 

Weight gain is a common symptom of hypothyroidism (low thyroid hormones). Therefore, one would assume that treating hypothyroidism would help a person lose weight. Surprisingly, taking thyroid medication (a synthetic version of T4 thyroid hormone, such as levothyroxine) helps to restore normal thyroid hormone levels but does not cause clinically significant weight loss.

A possible explanation is that obesity and hypothyroidism commonly occur together. As a result, patients and doctors assume that weight gain is due to hypothyroidism. However, this may not be true, and a person’s obesity may be due to other factors. This would explain why taking levothyroxine for the treatment of hypothyroidism does not lead to weight loss.

How quickly can you lose weight on levothyroxine?

Approximately half of all patients with hypothyroidism lose a modest amount of weight, approximately 8-9 pounds (lbs) over a period of up to 24 months on levothyroxine. 

A study conducted at the Boston Medical Center in Massachusetts enrolled adults with newly diagnosed hypothyroidism and an initial TSH (thyroid-stimulating hormone) level of over 10 mIU/L. The researchers excluded patients with thyroid cancer, postsurgical hypothyroidism, or a history of radioactive iodine therapy, head and neck radiation therapy, congestive heart failure, eating disorders, kidney disease, liver disease, pregnancy, and prescription weight-loss medication use. 

TSH levels and initial body weight at diagnosis and up to 24 months after initiation of levothyroxine treatment were measured. Weight change was recorded when serum TSH fell below 5 milli-international units per liter (mIU/L).

The study included 101 patients with an average age of 48 years, 71% women, an average initial TSH of 18.3 mIU/L, an average initial BMI of 29.3 kilograms per m2 (kg/m2), and an average initial body weight of 175 lbs. 

After treatment with levothyroxine, the average TSH fell to 2.3 mIU/L, the average weight loss after 5 months was -0.2 lbs, and the average decrease in body mass index (BMI) was -0.1 kg/m2. 

Only 52% of patients had lost weight at the conclusion of the study after 24 months. The average weight loss was a modest 8-9 lbs. The study concluded that in people with primary hypothyroidism, treatment with levothyroxine sodium does not lead to clinically significant weight loss. 

Does levothyroxine cause appetite loss?

Levothyroxine can cause both appetite loss and increased appetite as a side effect. Taking this medication can, therefore, lead to weight gain or weight loss. Some studies have found that levothyroxine causes increased hunger, which can cause you to gain weight. On the other hand, levothyroxine can promote weight loss by increasing your resting energy expenditure (basal metabolic rate).

Does levothyroxine speed up your metabolism?

Thyroid hormones speed up metabolism. Therefore, taking levothyroxine tablets may lead to an increased metabolism rate. People who have been struggling with weight gain due to slowed metabolism may find that losing weight is easier after starting levothyroxine for treating hypothyroidism when combined with healthy lifestyle changes.

Can people with normal thyroid hormone levels take levothyroxine for weight loss?

People with normal thyroid hormone levels should never take levothyroxine to treat obesity or achieve an average body weight. In euthyroid patients (people with normal thyroid function), taking the typically prescribed dose of levothyroxine does not result in weight loss but can cause serious health problems. 

Taking high doses of levothyroxine can lead to too much thyroid hormone in the body. This can cause symptoms of overactive thyroid (hyperthyroidism), such as chest pain, abnormal heart rhythm, shortness of breath, increased sweating, sleep problems, nervousness, anxiety, confusion, and seizures. Depending on your age, it may also cause severe cardiovascular symptoms.

Very high doses of levothyroxine can cause serious side effects and overdose symptoms, especially if you take this thyroid medicine with weight loss medications such as phentermine.

What is the downside to taking levothyroxine?

Some of the downsides of taking levothyroxine include:

Dosage and Administration

You need to take levothyroxine tablets daily on an empty stomach 30-60 minutes before the first meal of the day.

Levothyroxine is a narrow therapeutic index drug. Finding the right dose can take time. Small changes in thyroid hormone levels can affect many bodily functions. Too little levothyroxine may not treat your symptoms of hypothyroidism. Too much levothyroxine may cause symptoms of overactive thyroid, such as irregular heartbeats and difficulty breathing. 

It can take several weeks for levothyroxine to have a noticeable effect. Do not stop taking levothyroxine if you don’t notice an immediate improvement in your hypothyroidism symptoms. This doesn’t mean the medicine isn’t working properly. Tell your doctor how you feel on the medicine, and they will adjust your dose accordingly.

Regular Blood Tests

Your doctor will order regular blood tests to check thyroid stimulating hormone (TSH) levels and adjust the dose of levothyroxine based on the results and symptom control.

Side Effects of Levothyroxine

Levothyroxine can cause adverse effects such as changes in appetite, weight gain or weight loss, headache, nausea, vomiting, hair loss, changes in menstrual periods, leg cramps, and heat intolerance. 

Rarely, taking levothyroxine can cause more dangerous side effects, including a serious allergic reaction. Seek emergency medical attention if you develop a skin rash, hives, itching, swelling, or difficulty breathing. 

Interactions

Interactions with other medicines and food can affect how well levothyroxine works. Follow your doctor’s dosing instructions carefully for effective treatment.