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Levothyroxine

Supportive Care

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What is levothyroxine?

Levothyroxine (also called Synthroid®) is a thyroid hormone used to treat hypothyroidism. Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone.

Levothyroxine is available as tablets of several different strengths. All are taken by mouth. Below is a list of the tablet strengths and their colors:

  • 0.025-mg peach tablet
  • 0.050-mg white tablet
  • 0.075-mg violet tablet
  • 0.088-mg olive tablet
  • 0.1-mg yellow tablet
  • 0.112-mg rose tablet
  • 0.125-mg brown tablet
  • 0.137-mg turquoise tablet
  • 0.15-mg blue tablet
  • 0.175-mg lilac tablet
  • 0.2-mg pink tablet

Levothyroxine is also available as a clear liquid given by vein (IV).

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Possible side effects

Although side effects from levothyroxine are not common, they can occur. Most of these side effects reflect a response to the thyroid replacement. Either the dose of medicine is not enough or is too high. Always tell your doctor if you have any of these side effects:

  • Rapid heartbeat
  • Weight loss
  • Chest pain
  • Tremor
  • Headache
  • Diarrhea
  • Feeling nervous, irritable
  • Trouble sleeping
  • Sweating
  • Changes in appetite
  • Stomach cramp
  •  Fever
  • Hair loss
  • Skin rash
  • Changes in menstrual cycle
  • Sensitive to heat or cold

These are the most common side effects, but there may be others. Please report all side effects to the doctor or nurse.

Symptoms of an allergic reaction include: rash, hives, itching, chills, fever, headache, muscle ache, shortness of breath, coughing, tightness in the throat, swelling of the face or neck.

Not all patients who take levothyroxine will experience these side effects. Common side effects are bolded, but there may be others. Please report all suspected side effects to your doctor or pharmacist.

Find more information on side effects.

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Tips for patients and families

Be sure to discuss all questions and instructions with your doctor or pharmacist. 

  • If you are taking levothyroxine for an underactive thyroid, it may be several weeks before you notice an improvement. Check with your doctor if your symptoms do not improve or if you develop the above side effects. You may need an increase or decrease in the medicine dose. You may have to take this medicine for the rest of your life. Do not stop taking it unless your doctor tells you to do so.
  • Before you take levothyroxine, tell your doctor and pharmacist if you have a history of lactose intolerance or allergy to tartrazine. Certain brands of levothyroxine tablets contain lactose (Levothroid®) or tartrazine (Synthroid®).
  • Do not switch brands of levothyroxine without telling your doctor.
  • Before you take levothyroxine, it is important to tell your doctor if you have diabetes, hardening of the arteries, heart disease, high blood pressure, kidney disease, or an underactive adrenal or pituitary gland.
  • If you have surgery, including dental surgery, tell the doctor or dentist that you are taking levothyroxine.
  • Before you take levothyroxine, tell your doctor if you are pregnant or breast-feeding.
  • Levothyroxine may affect the way other medicines work. These medicines include warfarin, prednisone, and diabetes medicines. Always tell your doctor if you are taking these medicines, or if you start taking any new medicine while taking levothyroxine.

Levothyroxine at home:

  • You should take levothyroxine before breakfast to decrease trouble sleeping at night.
  • If you cannot swallow tablets, you can crush the tablets and mix with a small amount of water, flavored syrup, breast milk, or a non-soy based formula. You may also place crushed tablets on food such as cereal or applesauce. You must drink all the liquid mix or eat all the food as soon as you add the medicine.
  • If you miss a dose, take that dose as soon as you remember as long as the next scheduled dose is more than 4 hours away. If your next dose is due in less than 4 hours, do not take the missed dose. Never take 2 doses at one (1) time.
  • Do not take iron products, aluminum- and magnesium-containing antacids (Mylanta®, Rolaids®), calcium carbonate (Tums®, Maalox®), simethicone, sucralfate, Kayexalate®, colestipol, or cholestyramine within 4 hours of taking this medicine.
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Always check the medication label. Many medicines look the same. Do not take any new medicine, including over-the counter medicines or supplements, without checking with your doctor or pharmacist. Talk to your doctor or pharmacist about what to do in the case of a missed dose, allergic reaction, or medical emergency. 

This information is not intended to be a complete guide and is not a substitute for talking with your provider. Medicine names, appearance, formulas, or availability may vary by clinic location. Always follow the instructions given by your care team.

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Reviewed: June 2024