It’s a lab-made version of the T4 hormone

Levothyroxine tablets are a synthetic (lab-made) version of the T4 hormone. They have the same effect as T4 hormone, which is produced by your thyroid gland. That is why they work so well when your thyroid hormone levels are low.

When you take a levothyroxine tablet, it is converted into T3, the active hormone that affects the rest of your body. T3 can also be obtained through medications such as liothyronine (Cytomel).

Some people have questioned whether replacing both T3 and T4 in hypothyroidism is beneficial, but studies show no benefit. When possible, the current recommendation is to take levothyroxine alone for hypothyroidism.