Seeing 100 mcg on a prescription label can sound like a large amount, especially if you are new to thyroid medication. So, is 100 mcg of levothyroxine a high dose? For many adults, 100 mcg is not considered an unusually high dose, but whether it is appropriate depends on the individual. Age, body weight, heart health, thyroid function, other medications, and blood test results can all affect the dose a person needs.

Levothyroxine is a synthetic form of thyroxine, or T4, the hormone normally produced by the thyroid gland. It is commonly prescribed for hypothyroidism, also called an underactive thyroid. Keep in mind that 100 mcg equals just 0.1 mg.

Is 100 Mcg of Levothyroxine a High Dose?

There is no single levothyroxine dose that is right for everyone. According to current Synthroid prescribing information, the average full replacement dose for otherwise healthy adults with hypothyroidism is about 1.6 mcg per kilogram of body weight per day. As an example, the prescribing information lists about 100 to 125 mcg per day for a 70 kg, or roughly 154-pound, adult.

That does not mean every 154-pound adult should take this amount. Older adults, people with cardiovascular disease, and patients with long-standing severe hypothyroidism may be started at a much lower dose. The dose is then adjusted gradually based on thyroid-stimulating hormone, or TSH, levels and the person's response to treatment.

Your healthcare provider may also adjust your dose if your weight changes, you become pregnant, you start or stop certain medications, or your thyroid function changes over time.

How to Take Levothyroxine

Knowing how to take levothyroxine consistently is important because food, supplements, and medications can affect how much of the drug your body absorbs.

Levothyroxine is generally taken once daily on an empty stomach, about 30 minutes to one hour before breakfast. MedlinePlus also advises taking tablets with a full glass of water. Calcium and iron supplements can interfere with absorption and generally should be taken at least four hours apart from thyroid hormone medication.

For additional patient information, see the MedlinePlus levothyroxine medication guide.

Try to take your medication in the same way each day. If you use other prescription drugs, over-the-counter medications, vitamins, or supplements, ask your pharmacist or healthcare provider whether they need to be separated from levothyroxine.

How Do You Know if Your Dose Needs to Change?

Blood tests are an important part of levothyroxine treatment. The American Thyroid Association notes that TSH is commonly checked about six to eight weeks after starting thyroid hormone or changing a dose because the body needs time to reach a new steady level.

Symptoms can also provide useful information. Too little thyroid hormone may leave you feeling tired, cold, or constipated. A dose that provides more thyroid hormone than your body needs may cause symptoms such as nervousness, a rapid heartbeat, feeling unusually warm, sweating, tremors, or unexplained weight loss.

You can read more about treatment and monitoring through the American Thyroid Association hypothyroidism guide.

Do not increase, decrease, or stop levothyroxine based on symptoms alone. Your healthcare provider can review your symptoms and thyroid test results to determine whether a dose adjustment is appropriate.

Filling a Synthroid Prescription

Levothyroxine is available as generic medication and under brand names such as Synthroid. If your healthcare provider has prescribed the brand-name medication, you can review Synthroid pricing and ordering information at Canada Drugs Direct.

Whichever product you use, take it exactly as prescribed and keep up with recommended thyroid blood tests. Do not stop or change a prescribed medication without speaking with your healthcare provider.

This article is for general educational purposes and does not replace professional medical advice.

FAQs

What is a normal dose of levothyroxine?

There is no universal normal dose. The appropriate amount depends on factors such as body weight, age, cardiovascular health, thyroid function, and TSH results. Some adults may need around 100 mcg daily, while others require substantially less or more.

Is 100 mcg of levothyroxine equal to 0.1 mg?

Yes. There are 1,000 micrograms in one milligram, so 100 mcg equals 0.1 mg.

How long after changing levothyroxine should TSH be checked?

TSH is commonly rechecked about six to eight weeks after starting treatment or changing a thyroid hormone dose. Your healthcare provider may recommend a different schedule depending on your circumstances.

Can I take calcium or iron with levothyroxine?

Calcium and iron can reduce levothyroxine absorption. They generally should be taken at least four hours apart from thyroid hormone medication. Ask your pharmacist or healthcare provider how to schedule your specific medications and supplements.