T3 vs T4 – What is the difference?
Both T4 and T3 are iodine-containing hormones produced by the thyroid gland. They differ chemically only in that T3 (triiodothyronine) contains three iodine particles and T4 (tetraiodothyronine) contains four. While T4 is more stable and decomposes less quickly, T3 is up to a hundred times more effective than T4.
The body is also able to convert T4 into T3. The thyroid gland mainly produces T4, which can also be seen as a kind of “storage form” and is constantly available in the blood. If necessary, the organism can fall back on this “store” and convert T4 into T3.For this reason, thyroid tablets for hormone replacement usually contain the more stable T4.
Both T3 and T4 are usually determined when the thyroid gland values are measured in the blood. If there is a slight hormone deficiency, often only the T4 value is initially reduced. With a pronounced deficiency, however, both values are usually too low.
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