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Synthroid: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on March 1, 2023.

1. How it works

  • Synthroid is a brand (trade) name for levothyroxine. Levothyroxine is a man-made form of thyroxine (also called T4), a naturally occurring hormone produced by the thyroid gland. Synthroid replaces missing thyroxine in people whose bodies do not produce enough thyroxine naturally.
  • Synthroid belongs to the class of medicines known as thyroid hormones.

2. Upsides

  • Used for the treatment of hypothyroidism (low thyroxine levels in the body). Replaces or supplements low or missing thyroxine.
  • Levothyroxine is recommended by American guidelines as the preferred treatment for hypothyroidism. Synthroid is a brand of levothyroxine.
  • Synthroid may also be used in the management of goiter and some thyroid cancers.
  • Synthroid is available as a generic under the name levothyroxine.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Heart palpitations, headache, hair loss, flushing, diarrhea, and menstrual irregularities in women.
  • May cause a decrease in bone mineral density when used long-term, increasing the risk of fractures. Postmenopausal women are most at risk.
  • Not to be used for the treatment of obesity or weight loss. In people with normal thyroid function, the usual dosage of Synthroid is ineffective for weight reduction. Larger dosages may produce life-threatening toxicity.
  • There is a fine line between taking too much Synthroid and too little. Overtreatment may cause detrimental cardiac effects (such as an increase in heart rate, precipitation of angina, or arrhythmias). The elderly and people with pre-existing cardiac disease are most at risk.
  • Synthroid may worsen blood glucose control in people with diabetes. An increase in the dosage of antidiabetic medications and/or insulin may be needed.
  • May not be suitable for some people including those with cardiovascular disease, clotting disorders, diabetes, and adrenal or pituitary gland problems. Should not be used to treat myxedema coma because the absorption of Synthroid is unpredictable during this condition.
  • May interact with several drugs including anticoagulants, antidepressants, amiodarone, lithium, and some contrast agents.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

  • Synthroid replaces missing thyroxine and is used to treat hypothyroidism and other conditions associated with low thyroid hormone levels. The required dosage varies from person to person.

5. Tips

  • Take on an empty stomach, at least 30 to 60 minutes before food, with a full glass of water.
  • Avoid administration of Synthroid within 4 hours of taking iron or calcium supplements or antacids. Foods such as soy and soybean-based infant formula, coffee, walnuts, and those containing fiber or grapefruit juice can interfere with absorption.
  • Take exactly as directed by your doctor. The dosage of Synthroid depends upon a person's age, body weight, the presence of heart disease, other medical conditions and medications, and the condition being treated. Do not increase or decrease the dosage without your doctor's advice. Levels of Synthroid may take 6-8 weeks to stabilize following a dosage change.
  • Contact your doctor if you experience chest pain, shortness of breath, leg cramps, vomiting, diarrhea, excessive sweating, heat intolerance, fever, or a skin rash while taking Synthroid.
  • Regular blood tests are usually needed while you are taking Synthroid. Make sure you keep all your appointments.
  • Inform your doctor or dentist that you are taking Synthroid before any procedure or dental work.
  • Children may need intensive monitoring to prevent over or under-dosage as both can have detrimental effects on development.
  • In people and children who have difficulty swallowing, Synthroid tablets may be crushed and then suspended in one to two teaspoonfuls of water and then immediately administered either by spoon or dropper.

6. Response and effectiveness

  • Only 40-60% of a dose of Synthroid is absorbed from the gastrointestinal tract. Absorption is increased by fasting and decreased by certain foods, in older age, and by some medications.
  • May take up to 4 to 6 weeks for blood levels of Synthroid to stabilize with regular dosing.

7. Interactions

Medicines that interact with Synthroid may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with Synthroid. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with Synthroid include:

  • amiodarone or other medications that affect iodine, such as radioactive iodine
  • amphetamines, such as dexamphetamine or phentermine
  • anticoagulants, such as warfarin
  • anticonvulsants such as carbamazepine, phenobarbital, or phenytoin
  • antidepressants, such as sertraline or anti-anxiety medications
  • aspirin
  • ciprofloxacin
  • estrogens and oral contraceptives
  • heart medications, such as digoxin, metoprolol, or propranolol
  • HIV medications (eg, atazanavir, indinavir, ritonavir, or saquinavir)
  • medications for diabetes, including insulin
  • medications that can affect the absorption of levothyroxine, such as antacids, calcium carbonate, cholestyramine, iron, orlistat sucralfate, sevelamer, or proton pump inhibitors
  • rifampin.

Note that this list is not all-inclusive and includes only common medications that may interact with Synthroid. You should refer to the prescribing information for Synthroid for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Synthroid only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: March 1, 2023.