Thyroid hormones + Rifampicin (Rifampin) - Drug Interactions

Clinical evidence,mechanism, importance and management

A woman with Turner’s syndrome,who had undergone a total thyroidectomy and who was being treated with levothyroxine 100 micrograms daily, had a marked fall in serum levothyroxine levels and free levothyroxine index with a dramatic rise in TSH levels when given rifampicin. However, no symptoms of clinical hypothyroidism developed, and drop in serum levothyroxine occurred prior to starting rifampicin, which may reflect clinical picture of an acute infection (See reference number 1). Another case describes a fall in TSH levels when rifampicin was discontinued (See reference number 2).

A possible reason for changes is that rifampicin, a potent enzyme inducer, can markedly increase metabolism of many drugs and thereby reduce their effects. Rifampicin has been found to reduce endogenous serum thyroxine levels in healthy subjects(See reference number 3) and possibly in patients (See reference number 2).

There seem to be no reports of adverse effects in other patients given both drugs and evidence for this interaction is by no means conclusive. Although rifampicin can affect thyroid hormones,it appears that healthy individuals can compensate for this. Since hypothyroid patients may not be able to compensate in same way, bear this interaction in mind if rifampicin is given to a patient taking levothyroxine.

Isley WL. Effect of rifampin therapy on thyroid function tests in a hypothyroid patient on replacement L-thyroxine. Ann Intern Med (1987) 107,517–18.

Nolan SR,Self TH, Norwood JM. Interaction between rifampin and levothyroxine. South Med J (1999) 92, 529–31.

Ohnhaus EE,Studer H. A link between liver microsomal enzyme activity and thyroid hormonemetabolism in man. Br J Clin Pharmacol (1983) 15, 71–6.