Once-daily T4 is fine for most people

Dear Dr. Roach: If levothyroxine has a long half-life (seven days), how does the pituitary gland provide fine control of the thyroid’s T4 hormone during a short time period? If there is such fine control between the pituitary and thyroid, manually trying to simulate that feedback mechanism by taking one pill a day (levothyroxine) seems impossible. If using liothyronine slow-release medication, how often in one day should it be taken to keep the T3 level stable?

J.R.

Answer: When a person is healthy, the thyroid produces T4, levothyroxine, which is slowly metabolized by the body at a rate of about 10% of the T4 in the body per day. Although the thyroid also produces a small amount of the more metabolically active form of thyroid hormone, triiodothyronine (T3), most T3 in the body is converted from T4 outside the thyroid. T3 is degraded much faster, about 75% per day.

There is no fine control of thyroid hormone over minutes or hours. The pituitary gland can stimulate the thyroid to make more hormone through thyroid stimulating hormone (TSH), but that process is slow both to turn on and shut off. Once-daily T4 is an effective way for most people to get their thyroid hormone. The active T3 will then be converted from the T4 the person is taking.

A few people have a poor ability to convert the active T3 from T4. In these situations, giving pharmacologic T3 is appropriate. However, the combination T3 and T4 preparations that are commercially available do not have the same ratio of hormones that humans have. The ratio of T4 to T3 in desiccated thyroid extract from Armour and others is approximately 4:1, whereas in humans, it should be approximately 14:1. Using desiccated thyroid extract once daily leads to having excess T3 during part of the day (right after taking the medicine) and not enough T3 once the T3 has been metabolized.

Unfortunately, there is no slow-release formulation of T3: the regular release formulation is also called liothyronine and sold under the brand name Cytomel. When T3 is prescribed along with T4, it is dosed twice daily, usually 2.5 to 5 micrograms, but the T4 component may be dosed once daily.