L-thyroxine (T4 or tetraiodo-L-thyronine) and lio-thyronine (T3 or triiodo-L-thyronine) are the natural hormones of the thyroid gland. T4 is a less active precursor of T3, which is the major mediator of physiological effect. In this chapter T4 for therapeutic use is referred to as levothyroxine (the rINN, see p. 83) rather than levothyroxine (the former usage).
For convenience, the term 'thyroid hormone' is used to comprise T4 plus T3. Both forms are available for oral use as therapy.
Calcitonin: see page 741.
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