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Synthroid (levothyroxine) is a thyroid medicine that is used to replaces a hormone normally produced by your thyroid gland to regulate the body’s energy and metabolism.


The drug is used to treat hypothyroidism, a condition in which the thyroid gland produces not enough hormone. It is also prescribed to reduce the size of an enlarged thyroid gland (goiter) and to treat thyroid cancer.


The tablet is taken orally in the morning on an empty stomach with a small amount of liquid. The tablets should be taken regularly. If one dose of tablets is missed, the dose should not be increased with the subsequent intake of tablets.

The dosage is determined by a doctor individually for each patient.

  • Hypothyroidism: the initial dose is 50-100 mcg per day with a gradual increase (by 50 mcg every 2-3 weeks) to the maintenance dose – 150-250 mcg per day. For children, the initial dose is 12.5-50 mcg / day. With congenital hypothyroidism, children aged 6-12 years – 4-5 μg / kg / day, children aged 13-18 years whose puberty is not completed – 2-3 μg / kg / day, with completed puberty – 1.7 mcg / kg / day (for children under the age of 6 years, this dosage form is contraindicated);
  • After surgery for a malignant thyroid tumor: adults – up to 300 mcg per day, children – an initial dose of 50 mcg, then100-150 mcg per day;
  • Complex treatment of hyperthyroidism: 50-100 mcg per day;
  • Euthyroid goiter and prevention of relapse after resection: adults – 100-200 mcg per day, children – 50-150 mcg per day.

Side effects

  • Central nervous system: dizziness, headache, weakness, fatigue, apathy, lethargy, irritability, nervousness, anxiety, drowsiness, difficulty falling asleep. In children, there is a pseudotumor of the brain (severe headaches);
  • Skin and subcutaneous tissue: allergic reactions (skin rash, itching), dry skin, hyperemia, puffiness, increased sweating. In rare cases, angioedema (Quincke’s edema) was reported;
  • Cardiovascular system: arrhythmia, tachycardia, blood pressure drops, ventricular hypertrophy;
  • Musculoskeletal system: muscle weakness, myalgia, muscle cramps of the lower extremities, tremors, chest pain. In rare cases, menopausal women reported a decrease in bone density;
  • Gastrointestinal tract: nausea, vomiting, diarrhea, constipation, change in appetite;
  • Metabolism: increase / loss of weight;
  • Others: dysmenorrhea, fever.

If you have any severe adverse reactions, including those not specified in this instruction, you should consult a doctor.


  • Hyperthyroidism of various genesis,
  • Acute myocardial infarction,
  • Untreated adrenal cortex insufficiency,
  • Hypersensitivity to levothyroxine.

Pregnancy and breastfeeding

During pregnancy and breastfeeding, Synthroid should be used under medical supervision. The use in combination with thyreostatic drugs during pregnancy is contraindicated in connection with an increased risk of developing hypothyroidism in the fetus.

Interaction with other drugs

  • Levothyroxine reduces the effect of insulin and oral hypoglycemic drugs, cardiac glycosides; and intensifies the effect of indirect anticoagulants (prothrombin time increases), tricyclic antidepressants;
  • Colestyramine, colestipol, aluminum hydroxide reduce plasma concentration due to inhibition of absorption in the intestine;
  • Phenytoin reduces the amount of levothyroxine bound to the protein and the concentration of thyroxine (T4) by 15 and 25%, respectively;
  • Estrogens increase the concentration of the fraction associated with thyroglobulin (effectiveness decreases);
  • The simultaneous use with anabolic steroids, asparaginase, clofibrate, furosemide, salicylates, tamoxifen, can cause pharmacokinetic interaction at the level of protein binding.
  • Amiodarone, aminoglutethimide, PASK, ethionamide, antithyroid drugs, beta-blockers, carbamazepine, chloral hydrate, diazepam, levodopa, dopamine, metoclopramide, lovastatin, somatostatin affect the synthesis, secretion, distribution and metabolism of the drug.
  • The simultaneous use with phenytoin, salicylates, furosemide (in high doses), clofibrate increases the concentration of levothyroxine in the blood.

Special instructions

The drug should be used extreme caution in patients with cardiovascular diseases (including coronary heart disease, heart failure, arterial hypertension). In such cases, Synthroid should be used in a low initial dose, with a gradual increase.

In elderly patients and those with a prolonged course of hypothyroidism, treatment should be started gradually.

With thyroid replacement therapy in patients with adrenocortical insufficiency, lack of adequate maintenance therapy with corticosteroids can lead to an acute adrenal crisis.

The drug should be used with caution in diabetes and diabetes insipidus.

When conducting a differential diagnostic test of thyroid suppression, it is recommended that patients with diabetes mellitus increase doses of antidiabetic agents.

In some cases, thyroid hormones can cause or aggravate the previous myasthenic syndrome.