Levothyroxine dosage, forms, and strengths

Levothyroxine sodium is the preferred treatment for underactive thyroid (hypothyroidism), a condition in which the thyroid gland does not produce enough thyroid hormones. While levothyroxine will not cure hypothyroidism, it acts as a synthetic version of the thyroid hormone thyroxine (T4), replacing the missing thyroid hormone. Healthcare providers also use high doses of levothyroxine to treat some types of thyroid cancer after surgery or radioiodine therapy. For most people, levothyroxine is a daily tablet they will take for years or a lifetime.

Levothyroxine forms and strengths

Levothyroxine, the generic formulation of Synthroid, is taken orally as a tablet or oral solution, and infusions are used in emergencies. Doses are very small and precise, so 12 different dosage strengths vary in tiny amounts.

• Tablet: 25 micrograms (mcg), 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg

• Oral solution: 13 mcg/mL, 20 mcg/mL, 25 mcg/mL, 30 mcg/mL, 37.5 mcg/mL, 44 mcg/mL, 50 mcg/mL, 62.5 mcg/mL, 75 mcg/mL, 88 mcg/mL, 100 mcg/mL. 112 mcg/mL, 125 mcg/mL, 137 mcg/mL, 150 mcg/mL, 175 mcg/mL, 200 mcg/mL

• Injection: 100 mcg/5 mL, 200 mcg/5 mL, 500 mcg/5 mL

Levothyroxine dosage for adults

The U.S. Food and Drug Administration (FDA) has approved levothyroxine for the treatment of hypothyroidism and certain types of thyroid cancers that grow in the presence of thyroid-stimulating hormone (TSH). The dose is taken orally and individualized according to the healthcare provider. It may take several weeks or months to establish the correct dose. Levothyroxine is safe for most people to take. However, people with adrenal gland insufficiency that has not been treated or corrected will not be given levothyroxine.

Levothyroxine injections are only given in life-threatening emergencies to treat myxedema coma, a severe form of low thyroid hormones. This is the only case where injections are used. They are given as intravenous infusions, and the dose is based on response according to a healthcare provider.

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Levothyroxine dosage for hypothyroidism Most people who take levothyroxine have an underactive thyroid. The condition has many causes, including pituitary gland issues, autoimmune disorders, iodine deficiency, thyroid growths, thyroid disease, and radiation therapy or surgery to the thyroid gland. Whatever the cause, an underactive thyroid does not produce sufficient quantities of thyroid hormones, triiodothyronine (T3) and thyroxine (T4). Since thyroid hormones regulate many processes throughout the body, people can experience several health problems, including high cholesterol, heart disease, anemia, fatty liver, pain, weight gain, low energy, depression, dry skin, and hair loss. Levothyroxine is a synthetic version of thyroxine, so healthcare providers use it as a hormone replacement therapy. Since hypothyroidism can be a long-term or lifelong condition, most people with an underactive thyroid can expect to take daily doses of levothyroxine for the rest of their lives. The body only needs a very small amount of thyroid hormone daily, so getting the correct dose can take some time. A healthcare provider will determine the right dose based on symptoms and blood tests. The first doses will be “full replacement doses” with enough levothyroxine to fix someone with no thyroid gland. That’s about 1.6 micrograms of thyroxine per kilogram of body weight, but some people may receive smaller starting doses. Doses will then be steadily reduced to find the dose that produces healthy levels of thyroxine and thyroid-stimulating hormone (TSH) without any symptoms. Standard levothyroxine dosage for adults: 50-200 mcg taken once daily Maximum levothyroxine dosage for adults: 300 mcg daily Levothyroxine dosage for thyroid cancer Some cancers, called thyrotropin-dependent or well-differentiated thyroid cancer, grow in the presence of TSH. Therefore, healthcare providers don’t prescribe levothyroxine as a replacement hormone for thyroid cancer, they prescribe it to suppress thyroid-stimulating hormone (TSH). TSH stimulates the thyroid to release thyroid hormones. When thyroid hormones are low, TSH goes up. If thyroid hormones are too high, TSH levels go down. That’s how the relationship works. People with well-differentiated thyroid cancer are usually treated with radioiodine therapy and the surgical removal of the thyroid gland. As with all cancers, some cancer cells may be missed. To prevent the cancer remnants from growing into new tumors, cancer doctors (oncologists) prescribe high-dose levothyroxine to shut down TSH production. Without TSH, the cancer remnants don’t grow. Doses will vary because some people are more at risk of recurrence than others. Levothyroxine dosage for children As with adults, infants and children with an underactive thyroid are primarily treated with levothyroxine. Doses will vary based on the child’s age, weight, blood test results, and symptoms. Cancer doctors also use it as a follow-up treatment for thyrotropin-dependent thyroid cancer, and like adults, will use high doses to decrease serum TSH and prevent recurrence. Not every child can swallow a tablet. Parents or caregivers can crush the tablet in a small amount of water to make administering the dose easier. Ask the pharmacist or other healthcare professional how to do this to ensure the proper dose.

Levothyroxine dosage restrictions

Levothyroxine requires dose adjustments for several different situations or medical conditions. These include:

• Heart disease or seniors: The starting dose is lowered to 12.5–25 mcg/day, though the eventual dose will be the same as it would be in any adult

• Uncorrected adrenal insufficiency: Contraindicated

• Pregnancy: Doses usually increase by 30-50%; postpartum doses return to normal after delivery

• Newborns at risk of heart failure: Lower dose to start

• Children at risk for hyperactivity: Lower dose to start with gradual increases 

• Malabsorption syndromes: People who have problems absorbing nutrients through the intestines may require larger doses

• Renal impairment: No dosage modifications

• Hepatic impairment: No dosage modifications

Levothyroxine dosage for pets

As in people, levothyroxine is the preferred treatment for hypothyroidism in dogs and cats. Human levothyroxine tablets are too small for dogs and cats because their digestive systems don’t absorb oral levothyroxine as well as human digestive systems. Instead, Veterinarians use higher-dose veterinary formulations that are more appropriate for the animal. After an initial starting dose, veterinarians modify it by taking blood tests and reviewing the pet’s symptoms until they arrive at a working dosage.

How to take levothyroxine

Levothyroxine tablets are taken once per day. 

• Take your medicine as directed. Your dose may need to be changed several times to find what works best.

• Take each day’s dose in the morning on an empty stomach 30–60 minutes before breakfast.

• For children or infants who cannot swallow a tablet:

  • Crush the tablet and mix it in 1 to 2 teaspoons (5 to 10 mL) of water but not with any other liquid, such as infant formula or juice.

  • Immediately give the mixture to the child using a spoon or dropper. Do not store the mixture for future use.

• Store levothyroxine tablets in a closed container at room temperature, protected from moisture and light.

• For levothyroxine oral solution:

  • Levothyroxine oral solution can be taken directly in the mouth. 

  • Tirosint Sol can be taken directly in the mouth or by diluting it in some water.

  • Do not mix the oral solution with any other liquid besides water.

  • For oral solution in a bottle, measure the dose using a calibrated oral syringe only.

  • Ermeza comes with two oral syringes, one for small and another for large doses—use the appropriate syringe.

  • Rinse the syringe when finished.

  • For an oral solution that comes in an ampule (Tirosint Sol), take only the number of ampules prescribed by the healthcare provider.

  • Do not open the ampule until you’re ready to take or give a dose.

  • Store levothyroxine oral solution at room temperature and protect it from light.

Levothyroxine dosage FAQs

How long does levothyroxine take to work?

It may take four to six weeks of levothyroxine dosing before people feel better. 

How long does levothyroxine stay in your system?

The body eliminates half a dose of levothyroxine in 7.5 days, and it will take about five weeks for the dose to be completely eliminated.

How long can you take levothyroxine?

For many, levothyroxine therapy is lifelong. There are no side effects or complications due to long-term use. 

Can you overdose on levothyroxine?

Yes. You can take too many tablets. People with healthy thyroid may take levothyroxine for weight loss or athletic performance. Still, excess thyroid hormone is not healthy, and a healthcare provider should see anyone with symptoms of a levothyroxine overdose. 

Symptoms of a levothyroxine overdose are identical to an overactive thyroid (hyperthyroidism). These symptoms include increased energy, restlessness, mood swings, anxiety, irritability, high blood pressure, feeling hot, difficulty concentrating, and trouble sleeping. Severe overdose can cause life-threatening adverse effects such as chest pain, confusion, fast or irregular heartbeats (arrhythmias), hallucinations, stroke, coma, or death.

What interacts with levothyroxine?

Many drugs change thyroid hormone levels, so doctors must perform regular blood tests and possibly adjust the levothyroxine dose to avoid drug interactions. These drugs include:

• Glucocorticoids

• Beta blockers 

• The barbiturate phenobarbital

• The seizure medications carbamazepine and phenytoin

• The diuretic furosemide

• Estrogen replacement therapy

• The heart rhythm drug amiodarone

• Heparin blood thinners

• The antibiotic rifampin

Doses may also need to be adjusted in people taking antidepressants, diabetes drugs, or blood thinners. 

Some drugs, foods, and supplements interfere with the body’s ability to absorb an oral dose of levothyroxine. Levothyroxine should be taken at least four hours before or four hours after you take or eat: 

• Grapefruit juice, soybean flour, walnuts, and foods high in dietary fiber

• Calcium supplements

• Iron supplements

• Fiber supplements

• Antacids

• Proton pump inhibitors 

• The ulcer medication sucralfate

• Gas reducers like simethicone

• The cholesterol-lowering medications colestipol and cholestyramine

• Orlistat, an obesity drug that interferes with the body’s ability to absorb fats

Is it safe to take levothyroxine during pregnancy?

Levothyroxine is safe to take during pregnancy. The levothyroxine dose will increase to meet the increased demands of the pregnancy. 

Is it safe to take levothyroxine when breastfeeding?

Levothyroxine is safe to take while breastfeeding. It does not affect lactation or harm a nursing infant.

Sources

• Clinical practice guidelines for hypothyroidism in adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association, AACE Endocrine Practice

• Ermeza levothyroxine sodium solution prescribing information, DailyMed (NIH National Library of Medicine)

• Levothyroxine, StatPearls

• Levothyroxine and cancer, 70 Years of Levothyroxine

• Levothyroxine sodium injection solution prescribing information, DailyMed (NIH National Library of Medicine)

• Levothyroxine sodium tablet prescribing information, DailyMed (NIH National Library of Medicine)

• Pharmacokinetics of total thyroxine in dogs after administration of an oral solution of levothyroxine, Journal of Veterinary Pharmacology and Therapeutics

• Thyquidity levothyroxine sodium solution prescribing information, DailyMed (NIH National Library of Medicine)

• Tirosint Sol levothyroxine sodium solution prescribing information, DailyMed (NIH National Library of Medicine)

• Transport of the thyroid hormones across the feline gut wall, Thyroid