Levothyroxine: Uses, Side Effects & Interactions

Levothyroxine is used to treat the effects of a failing thyroid. It's typically prescribed for life so work with a doctor to adjust your dose as necessary.

Levothyroxine is one of a class of replacement hormones used to help treat thyroid gland failure. This medication will help replace the hormones that are absent from your system because they are not being secreted properly by the thyroid. This will help patients regulate their metabolism and energy level at a normal level. Though this medication can be used to help balance the hormones from your glandular system, it cannot assist in the management of any side effects of this condition, including hair loss or weight gain.

Indications and Usage

Levothyroxine is more commonly known by the brand names Levothroid, Levoxyl, Unithroid, and Snythroid. These drugs are offered in tablet form and should be taken first thing in the morning. You will need to take your dose on an empty stomach to ensure that it is absorbed properly. Take your tablet with a full glass of water, as these are sticky and tend to get caught in the throat and may cause choking or gagging when swallowing.

You should not stop taking levothyroxine without your doctor's knowledge and consent. Stopping this medication suddenly could lead to a withdrawal effect in the body, which amplifies your chances of developing side effects to the medication. It may take several weeks before you begin to see a change in your condition, so it is important that you take your medication regularly and do not alter your dosage until you have a chance to get an accurate rating of your condition.

Levothyroxine Dosage

Doses of levothyroxine will be adjusted over time based on the overall function of the thyroid gland and how well your body is responding to treatment.

  • Adults who have never been treated for thyroid conditions before will typically be started on 100-125 mcg per day, which will be increased after 4-6 weeks on the medications to help monitor whether or not the medication is taking effect properly.
  • The minimum effective dose for adults is 25 mcg per day. Doses should not be increased beyond 200 mcg per day.
  • Seniors and those with a history of cardiac disease will be restricted to 25-50 mcg per day until it has been shown that the patient can handle larger doses at a safe rate.
  • Children will be started on 12.5 mcg per day, which can be increased every 2-4 weeks as necessary. This will depend on the child's weight and the severity of their hypothyroidism. Children are typically given a liquid form of this medication rather than a tablet. In this case, doses will be restricted to 5-10 mL or 1-2 teaspoons of medication which will need to be dissolved in water. Newborns requiring levothyroxine will be given 10-15 mcg per day, which will be increased every 4-6 weeks depending on the necessity and evaluation of their cardiac health.

Levothyroxine Side Effects

Side effects vary from person to person, which usually include the following cases.

Common Side Effects

Circulation issues including neck pain, dilated veins or increased blood pressure and pulse may occur while on levothyroxine. Monitor your condition carefully and inform your regular doctor if these conditions escalate beyond a level that is considered safe. If your heart ever begins to race or you become extremely lightheaded, contact emergency medical services to avoid a potentially fatal reaction.

Other common side effects to levothyroxine include swelling in the neck and face, labored breathing, difficulty swallowing, heat intolerance, menstrual changes, nausea or irritability. In most cases these are a reaction to your body altering its hormone levels and the size of your thyroid. They should relax within a few days of starting your prescription. Do not stop taking your medication if these occur. If your difficulty breathing or swelling becomes life threatening or makes you feel as though you will lose consciousness, contact emergency medical services immediately.

Rare Side Effects

Rare side effects include eye pain, double or blurred vision, limping, pain in the legs, seizures, headaches and dizziness. These side effects will require medical attention, but are not life threatening. Contact your regular doctor to report these side effects and to get advice on how to treat them. An adjustment in your dosing size may be necessary to avoid any further negative reactions. Do not stop taking your medication all together unless expressly told to do so.

In less common cases, patients have reported clammy skin, confusion, lightheadedness, hives or rash. These may be signs of an allergic reaction or overdose on the medication. If these symptoms are paired with a shortness of breath or facial swelling, or they appear to come on suddenly after taking your medication you will need to contact emergency medical services or the FDA MedWatch center as quickly as possible to determine how to handle the situation properly.

In Children and Pregnant or Breastfeeding Women

Children may see an interruption of their natural growth patterns while on levothyroxine. They will need to have constant monitoring to ensure that they are not developing a condition that could cause permanent damage to the body. Pregnant women and those who are breastfeeding have not reported any damage to themselves or their babies while taking levothyroxine. You may need to adjust your dose to compensate for natural hormone changes that take place during this time, so talk with your doctor about how this may affect your dosing schedule.

Who Should Not Take It

Those who suffer from adrenal gland issues, heart or circulatory disease, have suffered a heart attack or thyrotoxicosis may not be able to take levothyroxine. This medication may increase your risk of having a heart attack or may cause an increase in the severity of your condition. Talk with your doctor about your heart health before starting on a prescription.


This medication is known for interacting with oral insulin. While it is safe to consume both medications at the same time, you will need to adjust your dosage of both substances to ensure that both are absorbed by your body properly. Dopamine agonists, aminoglutethimide amiodarone, clofibrate, furosemide, carbamazepine, antidepressants, cardiac glycosides or chloral hydrate may cause an adverse reaction by altering the hormones in the body. You will need to discuss how these medication combinations might affect your ability to take levothyroxine doses.

Levothyroxine has been shown to falsely alter laboratory tests for liver function or osteopenia. Inform your doctor that you are taking this medication so they can anticipate such changes in your results and form an alternate plan for performing these procedures. Some foods such as gluten may affect the function of your thyroid, causing you to require larger doses of levothyroxine. Talk with your doctor about how it may be necessary to adjust your diet to help ensure the highest possible level of thyroid health.



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