Question:

Dear Pharmacist,

What is the difference between Levothyroxine and Thyroxine? Can I use it interchangeably if one is not available? Is it equivalent in dosage?

Answer:

Your body produces two types of thyroid hormones, triiodothyronine (abbreviated T3), and thyroxine (abbreviated T4). The thyroid gland produces more T4 than T3, but the T3 is more biologically active. T4 can be converted to T3 in order to provide the body with the more active thyroid hormone. Your body can produce too much or too little of these hormones for a wide range of reasons.

When your body produces too little of these hormones, this condition is called hypothyroidism. Hypothyroidism can be treated with medications that are either animal-derived or synthetically-created thyroid hormones. Such medications can provide T3, T4, or both T3 and T4 in one tablet. Levothyroxine is a generic medication used to treat hypothyroidism that provides your body with synthetic T4 (thyroxine) which can be converted inside your body to T3.

On a chemical level Levothyroxine is an isomer of thyroxine which essentially means that levothyroxine molecules are chemically the same as thyroxine molecules but take on a different shape than thyroxine; both have the same effect in the body on metabolic function. Thyroxine preparations (including both L and D isomers) are currently not produced in the Unites States. Dosing of levothyroxine is typically based on thyroid-stimulating hormone levels in the blood.  Your doctor can help find a dose that will provide you with sufficient amounts of T4 (thyroxine).

Submitted by Sarah Mitchell, PharmD Candidate
University of Toledo College of Pharmacy

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