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Levothyroxine

Synonyms: L-thyroxine, LT4

Levothyroxine is the most important thyroid hormone. It acts predominantly as a prohomone. Its main (albeit not its sole) effect is mediated via its metabolic product Triiodthyronine (T3).

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 the 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.

Reference values

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 with the applied method.

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