Levothyroxine (Synthroid®, Levoxyl®, Tirosint®, Unithroid®)


Levothyroxine is a thyroid receptor agonist commonly prescribed for hypothyroidism.  It is a synthetically manufactured levo-isomer of the endogenous thyroxine (T4) hormone, and is converted peripherally to triiodothyronine (T3) hormone.  T3 binds to thyroid hormone receptors (TR) and can induce either transcriptional repression or derepression depending on the molecular and cellular context.  These transcriptional changes give rise to an increased sensitivity to catecholamines (e.g., raising heart rate and blood pressure) and increasing metabolic activity.  Pituitary thyroid-stimulating hormone (TSH) is downregulated by levothyroxine administration.

In addition to hypothyroidism resulting from Hashimoto’s thyroiditis and other causes, levothyroxine is also used to suppress TSH in other thyroid disorders.  Suppression of TSH can improve the clinical course of benign thyroid nodules and some thyroid cancers.

Levothyroxine should not be given in the presence of thyrotoxicosis (e.g., Graves’ disease) or pre-existing TSH suppression.  The most common side effects of levothyroxine involve GI distress, palpitations, tremor, and weight loss.  In rare circumstances, levothyroxine can also precipitate arrythmias and acute coronary syndromes.

In Plain English

Levothyroxine can replace thyroid hormone when your body does not make enough of it.  Thyroid hormone plays an important role in many processes throughout the body and insufficient amounts of it in your blood can lead to weight gain, lethargy, hair loss, and high cholesterol.

Clinical Considerations


Figure 1.  Levothyroxine clinical considerations at a glance


Evidence Basis

Levothyroxine in the News


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