MEDizzy - A 53-year-old woman who had undergone total thyroidectomy

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Drugs for Thyroid Disorders

A 53-year-old woman who had undergone total thyroidectomy started replacement therapy with levothyroxine. Which of the following pharmacological properties best explains why the levo isomer of the hormone is currently used instead of a racemic mixture for replacement therapy in thyroid disorders?

ExplanationThe use of the levo isomer of thyroxine is an example of the stereo selectivity of drugs. More than one half of all useful drugs are chiral molecules. Chirality (also called stereoisomerism) means that drugs can exist as enantiomer pairs; that is, they have asymmetric centers. A racemic mixture is a mixture of these enantiomers. In most cases, one of these enantiomers (dextro or levo) has greater affinity for the receptors than its mirror image; it fits better to the receptor molecule. However, most studies of clinical efficacy of drugs have been carried out with racemic mixtures of the drug rather than with separate enantiomers. Therefore, only a small percentage of the chiral drugs used clinically are marketed as the active isomers. The receptor affinity of levothyroxine is about 25 times higher than that of dextrothyroxine, and this is the reason why the levo isomer is used clinically.