What about T3?v
The thyroid gland is the sole source of T4 and the source of only 10 to 20% of T3. The remaining 80% to 90% of T3 comes from conversion of T4 to T3 by organs such as the liver, kidneys, brain, and skin. In other words, the body itself breaks down T4 to produce the amount of T3 it needs.
When a patient begins to take levothyroxine, the level of T4 rises and becomes stable over a five-week period. From then on, the level of T4 in the blood changes very little after each pill - the level of T4 is slow to rise and slow to fall. On the other hand, when a patient regularly takes T3, the level of T3 in the blood tends to rise and fall rapidly over a period of hours after each pill. In addition, since T3 is much more potent than T4, a patient may experience symptoms of hyperthyroidism for several hours after taking a substantial dose of T3.
Indeed, patients taking thyroid hormone preparations containing substantial amounts of T3 may prefer them to preparations containing only T4. Their reasons vary, but many patients say that it is easier to maintain their weight and that they have more energy when they take this combination of thyroid hormones. However, these benefits may come at a price - patients are often hyperthyroid for several hours each day. Hyperthyroidism predisposes these patients to osteoporosis, heart rhythm disturbances, and, possibly, a shorter life span. Substituting one disease for another is not desirable.
Commercially available thyroid hormone preparations made from animal thyroid glands invariably contain T3 and should not be used. Another reason to avoid these animal thyroid hormone preparations is the difficulty in preparing tablets that have exactly the same amount of thyroid hormones in each tablet. On the other hand, brand-name synthetic levothyroxine generally can be relied upon to have the stated amount of T4 in each tablet.
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