Pharmacokinetics of Levothyroxine

Distribution and Metabolism

• Age-related changes in body composition can affect plasma concentrations of levothyroxine, according to the European Heart Journal and Pharmacology and Therapeutics 1, 2
• Hepatic insufficiency can significantly affect the elimination of thyroid hormones, as reported by the European Heart Journal 1

Excretion and Pharmacokinetic Parameters

• Renal insufficiency has a smaller effect on plasma kinetics of thyroid hormones and their metabolites compared to hepatic insufficiency, as noted by the European Heart Journal and Dialogues in Clinical Neuroscience 1, 3
• Levothyroxine demonstrates linear pharmacokinetics at clinically relevant doses, according to Dialogues in Clinical Neuroscience 3, 4

Clinical Implications and Considerations

• Therapeutic drug monitoring may be useful when levothyroxine is used with medications that potentially interact with its metabolism, as suggested by the British Journal of Pharmacology 5

Common Pitfalls and Caveats

• Not accounting for age-related changes in pharmacokinetics may result in inappropriate dosing in elderly patients, as reported by the European Heart Journal 1