What Is Levothyroxine Used For – Side Effects and FAQs

Written by Phil Rossi, Laemsing Root PharmD

NowRx Pharmacy

women checked if levothyroxine used will work for her

Levothyroxine is a prescription medication used for treating hypothyroidism or low thyroid hormone. It is one of the most common prescription medications in the United States with over 100 million prescriptions written each year to more than 19 million patients. While there are several brand names for levothyroxine, some of the most popular brands include Synthroid, Levoxyl, Unithyroid, and Tirosint.

Let’s take a deeper look and explore what you need to know about the popular thyroid medication.

What Is Levothyroxine

Levothyroxine is a prescription medication that falls under the category of thyroid drugs. It is only available by prescription and helps support an underactive thyroid by replacing the T4 hormone in our body. While levothyroxine is the generic name for the medication, Synthroid, Levoxyl, Unithyroid, and Tirosint are all common brand names for the same drug. Your doctor will require a blood test before prescribing this medication as well as once you start taking it to see how well it’s working.

Levothyroxine is prescribed in a variety of strengths and comes either as a tablet or liquid you swallow. With over 100 million prescriptions written each year, this is one of the most common medications in the United States.

Is levothyroxine a controlled substance?

Levothyroxine is not a controlled substance. A controlled substance is a drug that is more tightly regulated by the government because of its potential for abuse or addiction. Under the Controlled Substances Act (CSA) the DEA placed each of these drugs into one of five schedules. You can find the full list of controlled substances in alphabetical order here.

Is levothyroxine the same as Synthroid?

Yes, levothyroxine and Synthroid are the same in terms of active ingredients. Synthroid is one of the most popular brand names for levothyroxine. While generic drugs may have slightly different inactive ingredients, the FDA requires that both brand name and generic medications pass rigorous standards for safety and efficacy. You can learn more about Synthroid through their website or by talking with your doctor.

What Is Levothyroxine Used For

Levothyroxine is a synthetic form of thyroxine used for replacing the T4 hormone in people with an underactive thyroid. This hormone is an important part of regulating metabolism and energy in our body. In other words, we need it for our body to function properly because it affects vital functions like body temperature, heartbeat, energy production, and even cognition.

Low levels of T4 have been linked to side effects including but not limited to fatigue and weight gain. According to the American Thyroid Association, common causes of hypothyroidism include autoimmune disease, radiation treatment, and surgical removal of the thyroid.

What does levothyroxine do?

Simply put, levothyroxine does what a functioning thyroid gland would do. Traditionally, the thyroid secretes T4 (and T3) hormones which bind to thyroid receptor proteins in the cell thereby increasing body metabolism via several mechanisms. However, hypothyroidism disrupts this process as little to no hormones are produced.

Fortunately, levothyroxine can mimic the body’s production of T4 by the thyroid. As a result, our body’s metabolic system can function normally even with an underactive or absent thyroid.

How To Take Levothyroxine

Generally, levothyroxine should be taken one-half to one hour before breakfast. In addition, it should be taken on an empty stomach at least 4 hours before or after drugs that are known to interfere with absorption.

Some brand name drugs such as Levoxyl suggest taking the medication with a full glass of water.

If you are taking any herbal supplements or over the counter medication, you should talk to your doctor before taking them with levothyroxine. Always talk to your doctor or pharmacist if you have any questions about your medication.

Can you take levothyroxine at night?

There is research that suggests you can take levothyroxine at night. A 2010 study found that levothyroxine taken at bedtime significantly improved thyroid hormone levels, while a follow up study in 2011 reaffirmed the same findings that there was no material difference between evening vs. morning dosage.

However, the consensus amongst medical professionals is that levothyroxine should be taken in the morning. If you believe an evening dose would be beneficial, have a conversation with your doctor. Do not make any changes to your medication regimen including dosage or time of dosage without first talking with a qualified healthcare professional.

What Are The Side Effects Of Levothyroxine

The main side effects of levothyroxine include but are not limited to the following:

  • Arrhythmias
  • Myocardial Infarction
  • Dyspnea
  • Muscle Spasm
  • Headache
  • Nervousness
  • Irritability
  • Insomnia
  • Tremors
  • Muscle Weakness
  • Increased Appetite
  • Weight Loss
  • Diarrhea
  • Heat Intolerance
  • Menstrual Irregularities
  • Skin Rash

Generally, these side effects are the result of overdosage. If you are experiencing side effects that will not go away, talk with your doctor about adjusting your dose or alternative options. If you believe you are having a medical emergency such as a heart attack, dial 911 for immediate medical assistance.

Keep in mind, that this is not a full list of side effects. You can find a more comprehensive list of side effects here or by talking with a trusted healthcare professional like your pharmacist or primary care physician.

Does levothyroxine cause hair loss?

Yes, levothyroxine can cause hair loss as a side effect. Thyroid problems have been linked to hair loss and since this medication replaces or augments our T4 production overdosing can result in hair loss. However, hair loss from this medication is typically the result of therapeutic overdosage. In other words, the dose you are taking may be too high. If you are experiencing hair loss as a side effect after starting your medication regimen, talk with your doctor about adjusting your dosage or alternative options.

Can levothyroxine cause acid reflux?

Acid reflux is not a side effect of levothyroxine. However, a positive correlation between dyspepsia (indigestion) and some thyroid issues has been observed in studies. In addition, increased weight gain due to an underactive thyroid can increase the risk for acid reflux symptoms. In other words, it may be a byproduct of your thyroid condition rather than the medication itself if you are experiencing heartburn after starting this medication regimen.

Does levothyroxine cause weight gain?

Levothyroxine does not generally cause weight gain and may actually cause weight loss as a side effect. An underactive thyroid affects our body’s ability to convert calories into energy which results in weight gain. Levothyroxine treats an underactive thyroid by replacing or producing more thyroid hormone which can cause weight loss as a byproduct.

If you are experiencing weight gain while taking this medication, you should talk with your doctor immediately as it could be a sign of a more serious adverse reaction. This medication should not be used for the treatment of obesity or weight loss.

How Long Does It Take Levothyroxine To Work

Levothyroxine starts working right away, however, it will take 4-6 weeks to reach maximal effectiveness. This will depend on a variety of factors that include age, body weight, cardiovascular status, medication conditions and medications being taken simultaneously, and the specific nature of the condition being treated.

In addition, your doctor will require a blood test once you start taking the medication and may need to adjust the dosage based on how well it’s working. Talk to your doctor if you have any questions or if you experience side effects while taking the medication.

How long does levothyroxine stay in your system?

Levothyroxine will stay in your system for up to 45 days. The half-life of levothyroxine or the time it takes the drug to reduce to half its original amount in your blood is about 6-7 days. After about 5 half-lives a drug is completely eliminated from the body. However, several medical conditions, foods, supplements, and medications will impact how long this medication stays in your body. Talk with your doctor or pharmacist if you have questions about your specific condition and medication or dosage.

Can You Take Levothyroxine With Other Medications

You can take levothyroxine with other medications, however, several drugs may impact its overall effectiveness and safety. For example, phosphate binders, proton pump inhibitors, and antacids may all affect absorption, thereby reducing the effectiveness of levothyroxine.

Talk with your doctor about any prescription or over the counter medications you may be taking to make sure there are no potential conflicts. In addition, your pharmacist or physician will be able to provide a complete list of drug interactions as it relates to levothyroxine.

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