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The majority of people with hypothyroidism are treated with one of the synthetic forms of the T4 thyroid hormone (Levoxyl, Synthroid). This is a more stable form of thyroid hormone and requires once a day dosing, whereas preparations containing T3 (the most active thyroid hormone) are much shorter-acting and need to be taken multiple times a day. Synthetic T4 is readily and steadily converted to T3 naturally in the bloodstream in the great majority of people, and this conversion is appropriately regulated by the body's tissues. A brand name preparation of L-thyroxine is recommended over generic preparations, and individuals should use the same brand of levothyroxine throughout treatment (See Figure 1 below).
Figure 1: The "rainbow" of L-thyroxine dose pills available. For patients taking L-thyroxine, blood levels of TSH should be checked every four to six weeks, in order to see if a L-thyroxine dose change is necessary
Other thyroid hormone replacements are available but are not often recommended for replacement therapy. These include: desiccated thyroid hormone, T3 (triiodothyronine), and combinations of thyroid hormones T3 and T4.
Stephanie L Lee, MD, PhD, FACE
Sonia Ananthakrishnan, MD
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