The Morning Journal (Lorain, OH)

Once-daily thyroid replacemen­t effective for most

- Contact Dr. Roach at ToYourGood­Health@med.

DEAR DR. ROACH >> If levothyrox­ine (the T4 hormone) has a long halflife (seven days), how does the pituitary gland provide fine control of the thyroid’s T4 hormone during a short time period (minutes, hours)?

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 slowreleas­e medication, how often in one day should it be taken to keep the T3 level stable?

— J.R.


When a person is healthy, the thyroid produces T4, levothyrox­ine, which as you correctly say 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 way there is with insulin, for example. 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 being taken.

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, because they’re made from pig thyroid. 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 commercial­ly available, 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, but the T4 component may be dosed once daily.

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