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Synonyms: L-thyroxine, LT4
Among others, thyroxine affects receptors in the CNS. Its plasma halflife is in the range of one week. The secretion rate of levothyroxine is controlled by TSH in the context of thyrotropic feedback control.
Due to the fact that the level of total L-thyroxine is determined not only by the thyroid's activity but also by the level of plasma proteins it is - with few exceptions - in diagnostical intention more reasonable to asses the levels of free levothyroxine rather than that of total hormone.
The reference interval for healthy adults amounts:
- Total L-Thyroxine: 64,4 to 154,4 nmol/l (5 to 12 mcg/dl)
- Free L-Thyroxine: 9 to 24 pmol/l (0,7 to 1,9 ng/dl)
Reference values may differ depending from the applied method.