Generic name: Levothyroxine.

Other brand names: Levothroid, Levoxine.

Synthroid is a synthetic thyroid hormone. It works exactly as the natural thyroid hormone produced in the body, regulating specific body functions.

Quick Facts About Synthroid


Used to treat any of the following conditions: thyroid hormone deficiency; at risk for or have developed a goiter; as a suppression test to determine if the thyroid is producing too much hormone; if you have had radiation therapy to treat or prevent thyroid gland cancer.

Synthroid Dosage

Take exactly as prescribed, not increasing or decreasing dose. Take dose at same time each day. If unable to swallow tablet, may mix into a spoonful of liquid or sprinkle over a small amount of food such as applesauce, and take right away. May take indefinitely if thyroid is not producing enough hormone. Doctor will monitor level of hormone through blood tests.

  • Usual adult dose: tailored to meet individual needs. Seniors, people with angina (crushing chest pain) — prescribed lower doses.
  • Usual child dose: tailored to meet individual needs.
  • Missed dose: take as soon as possible, unless almost time for next dose. In that case, do not take missed dose; go back to regular schedule. Do not double doses.

Synthroid Side Effects

Overdose symptoms:

abdominal cramps, chest pain, diarrhea, excessive sweating, fever, headaches, heat intolerance, irregular heartbeat, nervousness, palpitations, rapid heartbeat, tremors, trouble sleeping, weight loss. If you suspect an overdose, immediately seek medical attention.

More common side effects: other than overdose symptoms, side effects are rare. Children may lose hair, but this is rare. Excessive or rapid increase in dose may lead to overstimulation of thyroid gland. Overstimulation symptoms: abdominal cramps, changes in appetite, chest pain, diarrhea, fever, headache, heat intolerance, increased heart rate, irregular heartbeat, irritability, nausea, nervousness, palpitations, sleeplessness, sweating, tremors, weight loss.

No known less common or rare Synthroid side effects.


Inform your doctor before combining Synthroid with:

antacids such as Mylanta; antidiabetic medications such as Diabinese and Glucotrol; blood thinners such as Coumadin; cholestyramine medications such as Questran; colestipol (Colestid); epinephrine (EpiPen); estrogen or contraceptive pills such as Ortho-Novum and Premarin; insulin; lovastatin (Mevacor); phenytoin (Dilantin); tricyclic antidepressants such as Elavil and Tofranil. Blood test results may be altered if you are taking: androgens, corticosteroids such as Decadron and prednisone, estrogens such as Premarin, iodine-containing medications, oral contraceptives containing estrogen, or salicylate-containing medications such as aspirin.

No known food/other substance interactions.

Special Cautions

May take during pregnancy or breastfeeding if taking due to thyroid hormone deficiency.

Seniors are prescribed lower dose.

Follow doctor’s instructions carefully for children.

  • Not for use as a weight-loss medication. Overdose may cause life-threatening side effects, especially if taking with an appetite suppressant.
  • Doctor should not prescribe if: sensitive to or allergic to Synthroid; thyroid gland is overproducing thyroid hormone; or if adrenal glands are underproducing corticosteroid hormone.
  • If you have diabetes mellitus or diabetes insipidus, or if your body produces too little adrenal corticosteroid hormone, Synthroid will worsen your symptoms. Dosages for medications of these disorders will be adjusted during Synthroid therapy.

Note: provides accurate and independent information prescription pills, over-the-counter medicines and natural products. This material is provided for educational purposes only and is not intended for medical advice, diagnosis or treatment. If you have questions about  dosage, or Synthroid side effects, please contact your healthcare provider