How does this medication work? What will it do for me?

Levothyroxine is a thyroid hormone supplement for people who do not produce enough thyroid hormone on their own. Levothyroxine helps to reduce the symptoms of low thyroid hormone such as weight gain, sensitivity to cold, lack of energy, and dry skin. It may take several weeks for this medication to have a noticeable effect on your condition.

This medication is also used to decrease the size of an enlarged thyroid gland, also known as goiter.

Levothyroxine may also be used in addition to surgery and radioactive iodine treatment to treat certain types of thyroid cancer.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are being given this medication, speak to your doctor. Do not stop using this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to use this medication if their doctor has not prescribed it.

What form(s) does this medication come in?

Each single-dose vial contains 500 µg of levothyroxine sodium. Nonmedicinal ingredients: mannitol, tribasic sodium phosphate anhydrous, and sodium hydroxide.

How should I use this medication?

The dose of levothyroxine depends on how much of the hormone is needed to bring blood levels back to the normal range. This is determined by blood tests that are done in a laboratory. The starting dose will depend on your general physical condition and the severity and length of time that you have been experiencing symptoms of low thyroid hormone levels.

You should take this medication once a day at the same time every day to ensure a consistent effect. Ideally, it should be taken 30 minutes to 1 hour before eating breakfast, however it is most important that it be taken the same way every day. The dose of levothyroxine should be taken at least 4 hours before or after medications that change the way that levothyroxine is absorbed. Cotton seed meal, dietary fibre, soybean flour (infant formula), or walnuts may decrease the absorption of levothyroxine.

For infants or children who cannot swallow levothyroxine tablets, this medication may be crushed and added to a small amount (5 to 10 mL) of water, breast milk or non-soybean based formula. This mixture can then be given with a spoon or dropper. Tablets may also be crushed and sprinkled on a small amount of food like applesauce. Do not store crushed tablet mixtures for any period of time.

Signs that you may be getting too much thyroid hormone may include chest pain, increased heart rate, palpitations, excessive sweating, heat intolerance, and nervousness. If you think your dose of thyroid hormone may be too high, consult your doctor as soon as possible.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones given here, do not change the way that you are using the medication without consulting your doctor.

It is important to take this medication exactly as recommended by your doctor. If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, or if you miss 2 or more doses in a row, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not dispose of medications in wastewater (e.g. down the sink or in the toilet) or in household garbage. Ask your pharmacist how to dispose of medications that are no longer needed or have expired.

Who should NOT take this medication?

Do not take this medication if you:

  • are allergic to levothyroxine or any ingredients of the medication
  • are having a heart attack
  • have inflammation of the heart or heart muscles
  • have acute thyrotoxicosis (too much thyroid hormone in your system)
  • have uncorrected adrenal insufficiency
  • are pregnant and are taking medications for an overactive thyroid

What side effects are possible with this medication?

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

  • appetite changes
  • decreased fertility
  • diarrhea
  • fatigue
  • fever
  • flushing
  • hair loss (temporary; particularly in children during the first month of treatment)
  • headache
  • increased sweating
  • menstrual cycle changes
  • muscle cramps
  • muscle weakness
  • nervousness or irritability
  • restlessness
  • sensitivity to heat
  • sleepiness
  • stomach cramps
  • tremor (shaking)
  • trouble sleeping
  • upset stomach
  • weight changes

Although most of these side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

  • agitation
  • anxiety
  • joint pain, stiffness, or limping in children
  • mood swings
  • vomiting

Stop taking the medication and seek immediate medical attention if any of the following occur:

  • fast, pounding, or irregular heartbeat
  • signs of a heart attack (e.g., crushing chest pain radiating down left arm or jaw, nausea, vomiting, sweating, or shortness of breath)
  • signs of heart problems (e.g., fast, pounding, or irregular heartbeat, shortness of breath, swelling of the legs and ankles)
  • signs of increased pressure in the brain (children) (e.g., headaches, vision problems, ringing in the ears, arm pain)
  • seizures
  • severe allergic reaction (e.g., hives; difficulty breathing; wheezing; or swelling of the eyes, mouth, or lips)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Are there any other precautions or warnings for this medication?

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Bone density: Levothyroxine can cause bones to lose thickness. If you have an increased risk for osteoporosis (bone thinning) or are taking medications that reduce bone thickness (e.g., prednisone or antiseizure medications), before you start taking this medication, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Your doctor may monitor your bone thickness while you are taking this medication.

Diabetes: Levothyroxine raises blood sugar levels. For people with diabetes, this may result in an increase in the requirements for insulin or antidiabetes medications. If you have diabetes or are at an increased risk of developing diabetes, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

You may need to monitor your blood sugar more closely when starting this medication or changing doses of this medication.

Driving: Levothyroxine can affect the mental and physical abilities needed to drive or operate machinery safely. While you are taking levothyroxine, avoid driving, operating machinery, or performing other potentially hazardous tasks until you have determined how this medication affects you.

Heart disease: When starting levothyroxine, people with heart disease may be started on a lower dosage as it may cause the heart to work harder than it has been used to. If you have heart disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Seizures: Rarely, people taking levothyroxine have experienced seizures when they first start taking the medication. If you have a history of seizure disorder, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Signs of getting too much or too little medication: Contact your doctor as soon as possible if you notice any signs of getting too much medication (such as chest pain, confusion, fast or irregular heartbeat, mood swings, muscle weakness, psychosis, extreme restlessness, yellow eyes or skin, or shortness of breath) or signs of not getting enough medication (such as clumsiness, coldness, constipation, dry, puffy skin, listlessness, muscle aches, sleepiness, tiredness, weakness, or weight gain).

Weight loss: Levothyroxine should not be used for weight loss. Large doses of levothyroxine can cause serious or even life threatening side effects especially when taken together with other medications for weight loss.

Pregnancy: Levothyroxine should be taken throughout pregnancy to regulate the levels of thyroid hormone for the pregnant mother and the developing baby. If you become pregnant while taking this medication, contact your doctor as soon as possible. Your doctor may want to monitor your thyroid function more closely while you are pregnant.

Breast-feeding: Only a small amount of thyroid hormone is passed into breast milk. The use of appropriate amounts of this medication by breast-feeding women has not been shown to cause harm to breast-fed babies.

Seniors: Seniors may be more sensitive to the effects of levothyroxine.

What other drugs could interact with this medication?

There may be an interaction between levothyroxine and any of the following:

  • amiodarone
  • antacids (e.g., aluminum hydroxide, calcium carbonate, magnesium hydroxide)
  • anticonvulsants (e.g., carbamazepine, phenytoin, phenobarbital)
  • apalutamide
  • beta-blockers (e.g., metoprolol, propranolol)
  • birth control pills containing estrogen
  • calcium carbonate
  • calcium polystyrene sulfonate
  • chloroquine
  • cholestyramine
  • ciprofloxacin
  • colesevelam
  • colestipol
  • corticosteroids (e.g., dexamethasone, hydrocortisone, prednisone)
  • diabetes medications (e.g., glyburide, insulin, metformin, rosiglitazone)
  • diazepam
  • diet pills
  • digoxin
  • estrogens (e.g., conjugated estrogen, estradiol, ethinyl estradiol)
  • furosemide
  • heparin
  • iodide
  • interon-alfa
  • iron supplements (e.g., ferrous fumarate, ferrous gluconate, ferrous sulfate)
  • lithium
  • magnesium supplements (e.g., magnesium hydroxide, magnesium oxide)
  • maprotiline
  • metoclopramide
  • methimazole
  • multivitamins/minerals with iron, folic acid
  • nicotinic acid
  • non-steroidal anti-inflammatory medications (NSAIDs; e.g., diclofenac, ibuprofen, ketorolac, naproxen)
  • octreotide
  • orlistat
  • propylthiouracil
  • proton pump inhibitors (PPIs; e.g., esomeprazole, lansoprazole, omeprazole)
  • raloxifene
  • rifampin
  • ritonavir
  • selective serotonin reuptake inhibitors (SSRIs; e.g., citalopram, fluoxetine, paroxetine, sertraline)
  • semaglutide
  • sevelamer
  • simethicone
  • sodium polystyrene sulfonate
  • somatropin
  • certain statins (e.g., lovastatin, simvastatin)
  • sucralfate
  • testosterone
  • theophyllines (e.g., aminophylline, oxtriphylline, theophylline)
  • tamoxifen
  • thiazide diuretics (e.g., hydrochlorothiazide)
  • tricyclic antidepressants (e.g., amitriptyline, imipramine)
  • warfarin

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

  • stop taking one of the medications,
  • change one of the medications to another,
  • change how you are taking one or both of the medications, or
  • leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications that you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.

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