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Hypothyroidism occurs when the thyroid does not produce enough of the hormone thyroxine, sometimes referred to as T4.

NYU Langone doctors treat most people who have hypothyroidism using the hormone medicine levothyroxine, also referred to as T4. Levothyroxine supplements the body's lack of thyroxine. By taking levothyroxine, people who have hypothyroidism can restore their thyroid hormone levels to a normal range, thereby alleviating the symptoms associated with the condition.

Triiodothyronine, or T3, is the active form of thyroid hormone. Very small amounts are produced by the thyroid gland, but most T3 is produced through conversion of thyroxine into triiodothyronine in the body’s tissues. Sometimes, doctors prescribe the medicine liothyronine, which is a form of T3, to supplement low levels of triiodothyronine.

Though most people only require levothyroxine to treat their hypothyroidism, a small number of patients may need a combination of levothyroxine and liothyronine to feel their best.

Thyroid hormone replacement may be needed only for a short time if hypothyroidism is caused by a temporary condition, such as some forms of thyroiditis.

In people who have permanent conditions, such as Hashimoto’s thyroiditis or the removal of all or part of the thyroid gland—called thyroidectomy—thyroid hormone replacement is prescribed for life. Your doctor determines the dose based on follow-up blood tests. Thyroid medications are taken daily by mouth.

Levothyroxine

Most people easily tolerate levothyroxine. In fact, many report feeling better within just one week of beginning treatment.

If you have heart disease, your doctor may start you on a lower dose of levothyroxine, increasing it slowly while monitoring you. Too much of this medication can increase heart rate and the risk of arrhythmia, especially in older people, and overuse of levothyroxine contributes to bone loss.

Whether you have just started treatment or have been taking levothyroxine for years, it’s important to have your thyroid-stimulating hormone, or TSH, level monitored every six to 12 months—or more often if the level is not where it should be. Be sure to tell your doctor about any changes in your symptoms. Repeat thyroid tests and dose adjustments may be necessary.

Doctors typically advise treatment if the TSH level remains elevated and doesn’t resolve on its own or if symptoms are significant. If your blood test shows a mildly elevated TSH level and you do not have hypothyroid symptoms, your doctor may not recommend treating it immediately. Instead, your doctor may observe TSH levels for some time before prescribing medication.

Liothyronine

A small number of people who have hypothyroidism may never feel completely well on standard levothyroxine therapy. They may feel sluggish, experience mental cloudiness, or still struggle with weight gain. While taking levothyroxine typically results in normal blood levels of thyroxine, some people may be unable to convert thyroxine to the active thyroid hormone triiodothyronine, resulting in these symptoms. At NYU Langone, our doctors may prescribe levothyroxine in addition to liothyronine for these patients.