Description and Brand Names

Drug information provided by: Merative, Micromedex®

US Brand Name

  1. Synthroid

Descriptions


Levothyroxine injection is used to treat myxedema coma (severe hypothyroidism), a condition where the thyroid gland does not produce enough thyroid hormone. The powder form of levothyroxine injection can be used as a substitute for the oral dose when a rapid effect is needed and when the oral route is not allowed.

This medicine is to be given only by or under the supervision of your doctor.

This product is available in the following dosage forms:

  • Powder for Solution

Before Using

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For this medicine, the following should be considered:

Allergies

Tell your doctor if you have ever had any unusual or allergic reaction to this medicine or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.

Pediatric

Appropriate studies performed to date have not demonstrated pediatrics-specific problems that would limit the usefulness of levothyroxine injection in the pediatric population.

Geriatric

Appropriate studies performed to date have not demonstrated geriatric-specific problems that would limit the usefulness of levothyroxine injection in the elderly. However, elderly patients are more likely to have age-related heart and blood vessel problems, which may require caution in patients receiving levothyroxine injection.

Breastfeeding

Studies in women suggest that this medication poses minimal risk to the infant when used during breastfeeding.

Drug Interactions

Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are receiving this medicine, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.

Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

  • Apixaban
  • Dabigatran Etexilate
  • Digitalis
  • Edoxaban
  • Furosemide
  • Iopromide
  • Ketamine
  • Midodrine
  • Rivaroxaban
  • Warfarin

Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

  • Chlorotrianisene
  • Cholestyramine
  • Chromium
  • Colesevelam
  • Conjugated Estrogens
  • Conjugated Estrogens Synthetic A
  • Conjugated Estrogens Synthetic B
  • Dexlansoprazole
  • Dienestrol
  • Diethylstilbestrol
  • Esomeprazole
  • Esterified Estrogens
  • Estradiol
  • Estramustine
  • Estriol
  • Estropipate
  • Ethinyl Estradiol
  • Fosphenytoin
  • Imatinib
  • Kelp
  • Lansoprazole
  • Lanthanum Carbonate
  • Lopinavir
  • Mestranol
  • Omeprazole
  • Pantoprazole
  • Phenytoin
  • Polyestradiol Phosphate
  • Promestriene
  • Quinestrol
  • Rabeprazole
  • Rifampin
  • Ritonavir
  • Sevelamer
  • Simvastatin
  • Tibolone

Other Interactions

Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Using alcohol or tobacco with certain medicines may also cause interactions to occur. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.

Using this medicine with any of the following is usually not recommended, but may be unavoidable in some cases. If used together, your doctor may change the dose or how often you use this medicine, or give you special instructions about the use of food, alcohol, or tobacco.

  • Dietary Fiber
  • Grapefruit Juice
  • Soybean
  • Walnut

Other Medical Problems

The presence of other medical problems may affect the use of this medicine. Make sure you tell your doctor if you have any other medical problems, especially:

  • Adrenal insufficiency or Addison's disease (adrenal gland problem), uncorrected or
  • Thyrotoxicosis (overactive thyroid)—Should not be used in patients with these conditions.
  • Clotting disorder or
  • Diabetes or
  • Heart disease (history of) or
  • Underactive pituitary gland—Use with caution. May side effects to become worse.

Proper Use

A nurse or other trained health professional will give you this medicine in a medical facility. It is given through a needle placed into one of your veins.

You will receive a few doses of this medicine, then your doctor may switch you to an oral dosage form.

Dosing

The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.

  • For injection form (powder for solution):
    • For the treatment of hypothyroidism:
      • Adults—50 to 100 micrograms (mcg) injected into a muscle or vein once a day. People with very serious conditions caused by too little thyroid hormone may need higher doses.
      • Children—Dose is based on body weight and must be determined by your doctor.
  • For injection form (powder for solution):
    • For myxedema coma:
      • Adults—The loading dose is usually 300 to 500 micrograms (mcg) injected into a vein once a day. The maintenance dose is usually 50 to 100 mcg injected into a vein once a day. Your doctor may adjust your dose as needed and tolerated.
      • Children—Use and dose must be determined by your doctor.

Precautions

It is very important that your doctor check your progress at regular visits to allow your doctor to see if the medicine is working properly and to decide if you should continue to take it. Blood tests will be needed to check for unwanted effects.

This medicine may increase your risk of heart rhythm problems or heart attack. Check with your doctor right away if you have dizziness, fainting, fast, slow, or irregular heartbeat, chest pain or discomfort, or pain or discomfort in the arms, jaw, back, or neck.

This medicine may cause new or worsening heart failure. Check with your doctor right away if you have more than one of these symptoms: chest pain, decreased urine output, dilated neck veins, extreme fatigue, irregular breathing, irregular heartbeat, swelling of the face, fingers, feet, or lower legs, tightness in the chest, trouble breathing, or weight gain.

Levothyroxine should not be used for the treatment of obesity or for the purpose of losing weight. This medicine is ineffective for weight reduction and when taken in larger amounts may cause more serious medical conditions.

Make sure any doctor or dentist who treats you knows that you are using this medicine. This medicine may affect the results of certain medical tests.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

Side Effects

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor or nurse immediately if any of the following side effects occur:

Rare

  1. Seizures

Incidence not known

  1. Fast heartbeat
  2. fever
  3. hives, itching, skin rash
  4. hoarseness
  5. irritation
  6. joint pain, stiffness, or swelling
  7. redness of the skin
  8. swelling of the eyelids, face, lips, hands, or feet
  9. tightness in the chest
  10. troubled breathing or swallowing

Get emergency help immediately if any of the following symptoms of overdose occur:

  1. Anxiety
  2. changes in menstrual periods
  3. chest pain, discomfort, or heaviness
  4. decreased urine output
  5. diarrhea
  6. dilated neck veins
  7. excessive sweating
  8. extreme tiredness or weakness
  9. fainting
  10. fast, pounding, or irregular heartbeat or pulse
  11. feeling of warmth
  12. hand tremors
  13. headache
  14. heat intolerance
  15. increased appetite
  16. leg cramps
  17. mood changes
  18. muscle weakness or spasm
  19. nausea
  20. nervousness
  21. pain or discomfort in the arms, jaw, back, or neck
  22. restlessness
  23. sensitivity to heat
  24. stomach cramps
  25. sweating
  26. trouble sleeping
  27. trouble sitting still
  28. vomiting
  29. weight gain or loss

Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.

Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.

Portions of this document last updated: April 01, 2024

Original article: https://www.mayoclinic.org/drugs-supplements/levothyroxine-injection-route/before-using/drg-20072053