Synthroid (Injection)

Levothyroxine injection is used to treat myxedema coma (severe hypothyroidism), which is a condition where the thyroid gland does not make enough thyroid hormone. The powder form of levothyroxine injection can be used as a replacement for the oral dose when a quick effect is necessary and when taking it by mouth is not possible.

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

It is very important that your doctor check your progress at regular visits to see if the medicine is working properly and to decide if you should continue taking 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, a fast, slow, or irregular heartbeat, chest pain or discomfort, or pain or discomfort in your 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 losing weight. This medicine does not work for weight reduction and taking 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 you have discussed them with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

In deciding to use a medicine, you and your doctor need to consider the risks of taking the medicine against the benefits it will provide. Here are some important points to discuss:

Allergies

Inform your doctor if you have ever experienced any unusual or allergic reactions to this medicine or any other medicines. Also, let your healthcare professional know if you have any other allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, carefully read the label or package ingredients.

Pediatric

Current studies have not shown any specific problems in children that would limit the usefulness of levothyroxine injection for this age group.

Geriatric

Current studies have not shown any specific problems in older adults that would limit the usefulness of levothyroxine injection. However, older patients are more likely to have heart and blood vessel issues related to aging, which may require caution when taking levothyroxine injection.

Breast Feeding

Research suggests that this medication poses minimal risk to infants when used during breastfeeding.

Interactions with Medicines

Some medicines should not be used together at all, while others may be used together even if there is a potential interaction. In these cases, your doctor may adjust the dose or take other precautions. It is especially important for your healthcare professional to know if you are taking any of the medicines listed below while receiving this medicine. The following interactions are significant and may not be a complete list.

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

• Furosemide
• Iopromide
• Midodrine

Using this medicine with any of the following may increase the risk of certain side effects, but using both medicines may be necessary for your treatment. If both are prescribed, your doctor may change the dose or how often you take one or both 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

Interactions with Food/Tobacco/Alcohol

Some medicines should not be taken around the time of eating certain foods, as interactions may happen. Using alcohol or tobacco with certain medicines may also lead to interactions. The following interactions are significant and may not include everything.

Using this medicine with any of the following may increase the risk of certain side effects, but may be unavoidable in some instances. If used together, your doctor may adjust the dose or how often you take this medicine, or provide special instructions regarding food, alcohol, or tobacco.

• Soybean

Other Medical Problems

Having other medical issues may affect the use of this medicine. Be sure to tell your doctor about any other medical problems, especially:

• Adrenal insufficiency or Addison's disease (adrenal gland issue), uncorrected or
• Thyrotoxicosis (overactive thyroid)—This medicine should not be used in these conditions.

• Clotting disorder or
• Diabetes or
• Heart disease (history of) or
• Underactive pituitary gland—Use with caution. Side effects may worsen.

Along with its necessary effects, a medicine may cause some unwanted effects. Not all of these side effects may occur, but if they do, they may need medical attention.

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

Rare

• Seizures

Incidence not known

• Fast heartbeat
• fever
• hives, itching, skin rash
• hoarseness
• irritation
• joint pain, stiffness, or swelling
• redness of the skin
• swelling of the eyelids, face, lips, hands, or feet
• tightness in the chest
• troubled breathing or swallowing

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

• Anxiety
• changes in menstrual periods
• chest pain, discomfort, or heaviness
• decreased urine output
• diarrhea
• dilated neck veins
• excessive sweating
• extreme tiredness or weakness
• fainting
• fast, pounding, or irregular heartbeat or pulse
• feeling of warmth
• hand tremors
• headache
• heat intolerance
• increased appetite
• leg cramps
• mood changes
• muscle weakness or spasm
• nausea
• nervousness
• pain or discomfort in the arms, jaw, back, or neck
• restlessness
• sensitivity to heat
• stomach cramps
• sweating
• trouble sleeping
• trouble sitting still
• vomiting
• 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.

In the U.S.

• Synthroid

Available Dosage Forms:

• Powder for Solution

Therapeutic Class: Thyroid Supplement

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.

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