By Medically reviewed by hellodoktor

Generic Name: Synthroid Brand Name(s): Generics only. No brands available.


What is Synthroid® (levothyroxine) used for?

Synthroid® is a thyroid medicine that replaces a hormone normally produced by your thyroid gland to regulate the body’s energy and metabolism. Levothyroxine is given when the thyroid does not produce enough of this hormone on its own.

Synthroid® treats hypothyroidism (low thyroid hormone). Synthroid® is also used to treat or prevent goiter (enlarged thyroid gland), which can be caused by hormone imbalances, radiation treatment, surgery, or cancer.

How should I take Synthroid® (levothyroxine)?

Take Synthroid® exactly as prescribed by your doctor. Follow all directions on your prescription label. Your doctor may occasionally change your dose. Do not take this medicine in larger or smaller amounts or for longer than recommended.

Synthroid® works best if you take it on an empty stomach, one-half to one hour before breakfast. Levothyroxine absorption is increased on an empty stomach. Follow your doctor’s dosing instructions and try to take the medicine at the same time each day.

If a child is using this medicine, tell your doctor if the child has any changes in weight. Levothyroxine doses are based on weight in children, and any changes may affect your child’s dose.

It may take several weeks before your body starts to respond to Synthroid®. Keep using this medicine even if you feel well. You may need to use levothyroxine for the rest of your life.

While using Synthroid®, you may need frequent medical tests. Tell any doctor or dentist who treats you that you are using this medicine.

How do I store Synthroid® (levothyroxine)?

Synthroid® is best stored at room temperature away from direct light and moisture. To prevent drug damage, you should not store Synthroid® in the bathroom or the freezer. There may be different brands of Synthroid® that may have different storage needs. It is important to always check the product package for instructions on storage, or ask your pharmacist. For safety, you should keep all medicines away from children and pets.

You should not flush Synthroid® down the toilet or pour them into a drain unless instructed to do so. It is important to properly discard this product when it is expired or no longer needed. Consult your pharmacist for more details about how to safely discard your product.

Precautions & warnings

What should I know before using Synthroid® (levothyroxine)?

Before using this drug, tell your doctor if:

  • You are pregnant or breastfeeding. This is because, while you are expecting or feeding a baby, you should only take medicines on the recommendation of a doctor.
  • You are taking any other medicines. This includes any medicines you are taking which are available to buy without a prescription, such as herbal and complementary medicines.
  • You have allergy with any of active or inactive ingredients of Synthroid® or other medications.
  • You have any other illnesses, disorders, or medical conditions.

Since thyroid hormone occurs naturally in the body, almost anyone can take Synthroid®. However, you may not be able to take this medicine if you have certain medical conditions.

Tell your doctor if you have recently received radiation therapy with iodine (such as I-131).

Levothyroxine is not expected to harm an unborn baby. If you become pregnant while taking Synthroid®, do not stop taking the medicine without your doctor’s advice. Having low thyroid hormone levels during pregnancy could harm both mother and baby. Your dose needs may be different during pregnancy.

Levothyroxine can pass into breast milk, but it is not expected to be harmful to a nursing baby. Tell your doctor if you are breast-feeding. Your dose needs may be different while you are nursing.

Avoid the following food products, which can make your body absorb less levothyroxine: infant soy formula, cotton seed meal, walnuts, and high-fiber foods.

Do not give this medicine to a child without medical advice.

Is it safe during pregnancy or breastfeeding?

There are no adequate studies in women for determining risk when using Synthroid® during pregnancy or while breastfeeding. Please always consult with your doctor to weigh the potential benefits and risks before taking Synthroid®. This Synthroid® is pregnancy risk category A, according to the US Food and Drug Administration (FDA).

FDA pregnancy risk category reference below:

  • A=No risk
  • B=No risk in some studies
  • C=There may be some risk
  • D=Positive evidence of risk
  • X=Contraindicated
  • N=Unknown

Side effects

What side effects can occur from Synthroid® (levothyroxine)?

Common side effects may include:

  • Leg cramps
  • Muscle weakness
  • Headache
  • Feeling nervous or irritable
  • Diarrhea
  • Skin rash
  • Mild hair loss

Call your doctor at once if you have a serious side effect such as:

  • Fast or irregular heart rate
  • Chest pain, feeling short breath
  • Fever, hot flashes, sweating
  • Feeling unusually cold
  • Weakness
  • Tiredness
  • Sleep problems (insomnia)
  • Memory problems
  • Feeling depressed or irritable
  • Muscle aches
  • Dryness of your skin or hair
  • Hair loss
  • Changes in your menstrual periods
  • Vomiting
  • Diarrhea
  • Appetite changes
  • Weight changes

Get emergency medical help if you have signs of an allergic reaction to Synthroid®: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Not everyone experiences these side effects. There may be some side effects not listed above. If you have any concerns about a side-effect, please consult your doctor or pharmacist.


What drugs may interact with Synthroid® (levothyroxine)?

Synthroid® may interact with other drugs that you are currently taking, which can change how your drug works or increase your risk for serious side effects. To avoid any potential drug interactions, you should keep a list of all the drugs you are using (including prescription drugs, nonprescription drugs, and herbal products) and share it with your doctor and pharmacist. For your safety, do not start, stop, or change the dosage of any drugs without your doctor’s approval.

Certain medicines can make levothyroxine less effective if taken at the same time. If you use any of the following drugs, avoid taking them within 4 hours before or 4 hours after you take levothyroxine:

  • Calcium carbonate (Alka-Mints, Calcium Oyster Shell, Caltrate, Os-Cal, Oyster Shell Calcium, Rolaids Soft Chew, Tums, and others)
  • Cholestyramine, colestipol
  • Ferrous sulfate iron supplement
  • Sucralfate
  • Sodium polystyrene sulfonate (Kalexate, Kayexalate, Kionex)
  • Antacids that contain aluminum or magnesium – Acid Gone, Gaviscon, Maalox, Milk of Magnesia, Mintox, Mylanta, Pepcid Complete, and others)

Does food or alcohol interact with Synthroid® (levothyroxine)?

Synthroid® may interact with food or alcohol by altering the way the drug works or increase the risk for serious side effects. Please discuss with your doctor or pharmacist any potential food or alcohol interactions before using this drug.

What health conditions may interact with Synthroid® (levothyroxine)?

Synthroid® may interact with your health condition. This interaction may worsen your health condition or alter the way the drug works. It is important to always let your doctor and pharmacist know all the health conditions you currently have.

Health conditions that may interact with this drug are:

  • A thyroid disorder called thyrotoxicosis
  • Heart disease, coronary artery disease, or a history of blood clots
  • Diabetes (insulin or oral diabetes medication doses may need to be changed when you start taking Synthroid®)
  • Anemia (lack of red blood cells)
  • Osteoporosis, or low bone mineral density
  • Problems with your pituitary gland
  • Any food or drug allergies
  • An untreated or uncontrolled adrenal gland disorder
  • A heart attack, or are having any symptoms of a heart attack (chest pain or heavy feeling, pain spreading to the jaw or shoulder, nausea, sweating, general ill feeling)


The information provided is not a substitute for any medical advice. You should ALWAYS consult with your doctor or pharmacist before using Synthroid® (levothyroxine).

What is the dose of Synthroid® (levothyroxine) for an adult?

Primary Hypothyroidism in Adults and in Adolescents in Whom Growth and Puberty is Complete

Start at the full replacement dose in otherwise healthy, non-elderly individuals who have been hypothyroid for only a short time (such as a few months).

The average full replacement dose of Synthroid® is approximately 1.6 mcg per kg per day.

Adjust the dose by 12.5 to 25 mcg increments every 4 to 6 weeks until the patient is clinically euthyroid and the serum TSH returns to normal.

Doses greater than 200 mcg per day are seldom required.

An inadequate response to daily doses of greater than 300 mcg per day is rare and may indicate poor compliance, malabsorption, drug interactions, or a combination of these factors.

For elderly patients or patients with underlying cardiac disease, start with a dose of 12.5 to 25 mcg per day. Increase the dose every 6 to 8 weeks, as needed until the patient is clinically euthyroid and the serum TSH returns to normal. The full replacement dose may be less than 1 mcg per kg per day in elderly patients.

In patients with severe longstanding hypothyroidism, start with a dose of 12.5 to 25 mcg per day. Adjust the dose in 12.5 to 25 mcg increments every 2 to 4 weeks until the patient is clinically euthyroid and the serum TSH level is normalized.

Secondary or Tertiary Hypothyroidism

Start at the full replacement dose in otherwise healthy, non-elderly individuals.

Start with a lower dose in elderly patients, patients with underlying cardiovascular disease or patients with severe longstanding hypothyroidism as described above.

Serum TSH is not a reliable measure of Synthroid® dose adequacy in patients with secondary or tertiary hypothyroidism and should not be used to monitor therapy.

Use the serum free-T4 level to monitor adequacy of therapy in this patient population.

Titrate Synthroid® dosing per above instructions until the patient is clinically euthyroid and the serum free-T4 level is restored to the upper half of the normal range.


Pre-existing Hypothyroidism:

Synthroid® dose requirements may increase during pregnancy.

Measure serum TSH and free-T4 as soon as pregnancy is confirmed and, at minimum, during each trimester of pregnancy.

In patients with primary hypothyroidism, maintain serum TSH in the trimester-specific reference range.

For patients with serum TSH above the normal trimester-specific range, increase the dose of Synthroid® by 12.5 to 25 mcg/day and measure TSH every 4 weeks until a stable Synthroid® dose is reached and serum TSH is within the normal trimester-specific range.

Reduce dosage to pre-pregnancy levels immediately after delivery and measure serum TSH levels 4 to 8 weeks postpartum to ensure dose is appropriate.

New Onset Hypothyroidism:

Normalize thyroid function as rapidly as possible.

In patients with moderate to severe signs and symptoms of hypothyroidism, start Synthroid® at the full replacement dose (1.6 mcg per kg body weight per day).

In patients with mild hypothyroidism (TSH < 10 IU per liter) start at 1.0 mcg per kg body weight per day.

Evaluate serum TSH every 4 weeks and adjust Synthroid® dosage until a serum TSH is within the normal trimester specific range.

TSH Suppression in Well-differentiated Thyroid Cancer

Generally, TSH is suppressed to below 0.1 IU per liter, and this usually requires a Synthroid® dose of greater than 2 mcg per kg per day.

However, in patients with high-risk tumors, the target level for TSH suppression may be lower.

What is the dose of Synthroid® (levothyroxine) for a child?

Pediatric Dosage – Congenital or Acquired Hypothyroidism

The recommended daily dose of Synthroid® in pediatric patients with hypothyroidism is based on body weight and changes with age.

Start Synthroid® at the full daily dose in most pediatric patients. Start at a lower starting dose in newborns (0-3 months) at risk for cardiac failure and in children at risk for hyperactivity.

How is Synthroid® (levothyroxine) available?

Synthroid® (levothyroxine) is available in the following dosage forms and strengths:

  • Tablet: levothyroxine sodium 300ug

What should I do in case of an emergency or overdose?

In case of an emergency or an overdose, call your local emergency services or go to your nearest emergency room.

What should I do if I miss a dose?

If you miss a dose of Synthroid®, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and take your regular dose as scheduled. Do not take a double dose.

Hello Health Group does not provide medical advice, diagnosis or treatment.

Review Date: July 19, 2017 | Last Modified: September 13, 2019

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