Levothyroxine (Injection)

When administered via injection, Levothyroxine is used to treat patients with very low levels of thyroid hormone or patients with an underactive thyroid who are unable to take medication orally.


When a patient's thyroid gland isn't functioning properly, it may not produce enough thyroid hormones. Known as hypothyroidism, patients can experience a range of symptoms as a result of this condition. As the thyroid hormones play a significant role in controlling the patient's metabolism, when the output of the thyroid gland is low, patients may experience weight gain, constipation, depression and fatigue.

Although an underactive thyroid can be caused by certain medical procedures, such as a thyroidectomy, it can also occur when the thyroid fails to produce hormones effectively. If the amount of naturally produced thyroid hormones is not sufficient, the patient's levels can be supplemented by medication.

Generally, patients with hypothyroidism are prescribed Levothyroxine in oral form. Taken on a daily basis, this is usually sufficient in treating the symptoms of an underactive thyroid. In some cases, however, Levothyroxine may need to be administered more quickly and, in these instances, Levothyroxine may be delivered via an injection. In addition to this, Levothyroxine can be administered via injection if patients are unable to take medication orally.

When patients are suffering from a severe thyroid hormone deficiency, they may enter a state known as a myxedema coma. This is a potentially fatal complication of hypothyroidism which can be precipitated by another medical event, such as a stroke, heart attack or infection. The most common symptom of decompensated hypothyroidism is a low body temperature. Following this, patients may also exhibit a slow heart rate, hypoxia, hyperventilation, low blood pressure and/or low blood sugar.

Although a myxedema coma is life-threatening, patients can be successfully treated with Levothyroxine. By administering Levothyroxine via intravenous injection, physicians can ensure that the medication is delivered to the patient's system as quickly as possible. Once it starts to take effect, the patient's hormone levels should rise and their symptoms should lessen.

Whilst hypothyroidism is a common condition and can often be controlled with oral medication, Levothyroxine injections are suitable for patients who are unable to take medication orally or who are suffering from serious complications of hypothyroidism. When used in these instances, intravenous Levothyroxine injections can be life-saving and can provide relief from debilitating symptoms.

Conditions Treated


  • Myxedema coma

Type Of Medicine

  • Synthetic thyroid hormone

Side Effects

Like most medications, Levothyroxine may cause patients to experience some side-effects. However, there are serious adverse effects which can occur if too much Levothyroxine is given to the patient. The symptoms of a Levothyroxine overdose may include:

  • Changes in appetite
  • Weight loss
  • Changes in menstrual periods
  • Vomiting
  • Chest pain
  • Trouble sleeping
  • Diarrhea
  • Sweating
  • Irregular or fast heartbeat
  • Shortness of breath
  • Irritability
  • Fever
  • Sensitivity to heat
  • Leg cramps
  • Hand tremors
  • Nervousness
  • Headache

If patients display the above side effects after being treated with Levothyroxine, it is vital that they receive emergency medical treatment as quickly as possible. When Levothyroxine is administered via intravenous injection, it is usually given in a clinical setting, such as a hospital or treatment center. Due to this, patients should be monitored during and following treatment and it should be easy to access medical assistance. If the symptoms of an overdose occur, the patient or their caregiver should notify a healthcare practitioner immediately. If patients are at home or out when these symptoms occur, they should call 911 or visit their nearest hospital.


If patients are prescribed Levothyroxine injections because they are unable to take medicine orally, they are usually instructed to administer 50-100mcg on a daily basis. This can be injected into a vein or muscle. Whilst a trained healthcare practitioner will need to administer injections into the patient's vein, patients may be able to administer their own intramuscular injections.

However, if patients are prescribed Levothyroxine injections because they are suffering from extremely low levels of thyroid hormones, they may be prescribed a higher dose of medication. If the medicine needs to be administered swiftly due to the patient's clinical presentation, it will normally be given via intravenous injection.

If patients are in a state of myxedema coma, for example, they may be given 300-500mcg via intravenous injection as an initial dose. Following this, the patient may be given 50-100mcg per day via intravenous injection. Once the patient's condition improves, they may be given Levothyroxine orally, as opposed to via an injection.

When patients are prescribed Levothyroxine injections, they should use the medication as their doctor has instructed them to. If the medication is being administered intravenously, a physician or nurse will calculate the patient's dose. However, if the patient is administering intramuscular injections of Levothyroxine, they will be taught how to calculate the appropriate dose and deliver the injection.

If patients are unsure how to use Levothyroxine or when to administer their medication, they should contact their physician or pharmacist for advice.

Potential Drug Interactions

As some medicines can interact with each other, Levothyroxine is not usually prescribed if patients are taking any of the following medications:

  • Amineptine
  • Melitracen
  • Amitriptyline
  • Mirtazapine
  • Amitriptylinoxide
  • Ketamine
  • Amoxapine
  • Opipramol
  • Clomipramine
  • Maprotiline
  • Desipramine
  • Protriptyline
  • Dibenzepin
  • Nortriptyline
  • Doxepin
  • Tianeptine
  • Imipramine
  • Trimipramine
  • Lofepramine

Although the use of Levothyroxine in conjunction with the above medicines is not usually advisable, it may be necessary in some cases. If so, the patient's dose of medicine may be modified or they may be advised to take their medication at a specific time as this could help to reduce the risk of an interaction occurring.

If patients use Levothyroxine whilst taking any of the following medicines, they may have an increased risk of developing side effects:

  • Acenocoumarol
  • Kelp
  • Anisindione
  • Imatinib
  • Cholestyramine
  • Pantoprazole
  • Chromium
  • Lopinavir
  • Colesevelam
  • Omeprazole
  • Conjugated Estrogens
  • Phenytoin
  • Dexlansoprazole
  • Rabeprazole
  • Lanthanum Carbonate
  • Dicumarol
  • Phenprocoumon
  • Eltrombopag
  • Rifampin
  • Esomeprazole
  • Phenindione
  • Esterified Estrogens
  • Lansoprazole
  • Rifapentine
  • Estradiol
  • Ritonavir
  • Estriol
  • Simvastatin
  • Estrone
  • Sevelamer
  • Estropipate
  • Warfarin
  • Fosphenytoin

As some medicines can interact with other substances, such as food and drinks, patients should know what to avoid when they are being treated with Levothyroxine. If patients consume the following whilst using Levothyroxine, they may suffer from increased side effects:

  • Soybean

Before using Levothyroxine, patients should tell their doctor if they are taking any other medicines, vitamins or supplements. Furthermore, patients should obtain medical advice before using any new medicines, supplements or vitamins once they have started treatment with Levothyroxine.


If patients have any other medical conditions or a history of certain conditions, it may affect their treatment with Levothyroxine. Due to this, patients should discuss their health and medical history with their physician before Levothyroxine injections are administered. The following conditions may be particularly relevant:

  • Heart disease
  • Diabetes
  • Clotting disorder
  • Underactive pituitary gland
  • Adrenal gland disorders
  • Thyrotoxicosis
  • Heart attack
  • Addison's disease or adrenal insufficiency

If patients are receiving Levothyroxine injections on a regular basis, they should also have regular consultations with their physician. In addition to this, frequent blood tests may be carried out. This will enable physicians to monitor the patient's levels of thyroid hormones and determine whether their dose of Levothyroxine needs to be adjusted.

Levothyroxine should not be used to facilitate weight loss. This medication will not promote weight loss and, if taken in large amounts, can cause serious health problems.

As Levothyroxine has been classified as a category A medication in terms of pregnancy, it is not thought to pose a significant risk to pregnant patients or their unborn baby. Due to this, pregnant patients may be treated with Levothyroxine, if it is necessary.

Similarly, studies have not shown that Levothyroxine presents a significant risk to infants if it is used by breastfeeding mothers. Although patients should obtain medical advice before breastfeeding whilst using Levothyroxine, it is not thought that this medication poses a risk to infants if it is excreted in breast milk.

Before using Levothyroxine, patients should tell their physician if they have any allergies or have ever exhibited an allergic reaction. In rare cases, patients may develop an allergic reaction after being treated with Levothyroxine. If so, patients will require urgent medical treatment. If patients experience an allergic reaction, they may display the following symptoms:

  • Itching
  • Shortness of breath
  • Difficulty swallowing
  • Wheezing
  • Hives
  • Rash on the skin
  • Hoarseness
  • Difficulty breathing
  • Swelling, affecting the hands, lips, face, tongue, throat or mouth


If Levothyroxine is being administered via an injection, it is usually supplied in powder form and should be reconstituted before use. Typically, the powder should be kept at a controlled room temperature of 20?-25?C (68?-77?F) and should be protected from light. However, patients should follow the manufacturer's specific instructions when storing this medication.

If patients are advised to keep Levothyroxine at home, they must store it in a safe location and ensure that children and/or pets cannot gain access to it.

When disposing of medical equipment, such as needles or syringes, patients should never discard these items along with normal household waste. Specialist medical waste boxes should be provided so that used needles and other equipment does not pose a danger to anyone. Patients should contact their physician's office or pharmacist and use a designated medical waste service.


Although hypothyroidism can cause debilitating symptoms and serious complications if untreated, patients can obtain relief from symptoms via treatment with Levothyroxine. If patients are unable to take Levothyroxine orally, the medication can be administered via an intravenous or intramuscular injection. When used regularly, the medication should supplement the patient's thyroid hormone levels and ensure that they remain at a stable level.

Similarly, Levothyroxine injections can be used if the patient's thyroid hormones drop to a dangerously low level. In such cases, intravenous Levothyroxine injections enable the medication to be administered quickly and can prevent the potentially fatal complications of extreme hypothyroidism.