Abstract
BackgroundLevothyroxine (LT4) and rifampin (RIF) are sometimes used together; however, no clinical studies have assessed the effects of these drugs on thyroid function or the need to adjust LT4 dose.MethodsWe retrospectively reviewed the records of 71 Korean patients who started RIF during LT4 treatment. Clinically relevant cases that required dose adjustment according to the American Thyroid Association (ATA)/American Association of Clinical Endocrinologists (AACE) guidelines were identified, and risk factors of increased LT4 dose were analyzed.ResultsAfter administering RIF, median serum thyroid-stimulating hormone (TSH) level (2.58 mIU/L, interquartile range [IQR] 0.21–7.44) was significantly higher than that before RIF (0.25 mIU/L, IQR, 0.03–2.62; P < 0.001). An increased LT4 dose was required for 50% of patients in the TSH suppression group for thyroid cancer and 26% of patients in the replacement group for hypothyroidism. Risk factor analysis showed that remaining thyroid gland (odds ratio [OR] 9.207, P = 0.002), the time interval between starting RIF and TSH measurement (OR 1.043, P = 0.019), and baseline LT4 dose per kg body weight (OR 0.364, P = 0.011) were clinically relevant variables.ConclusionsIn patients receiving LT4, serum thyroid function test should be performed after starting RIF treatment. For patients with no remnant thyroid gland and those receiving a lower LT4 dose, close observation is needed when starting RIF and TB medication.
Highlights
Levothyroxine (LT4) is widely used to replace thyroid hormone in patients with hypothyroidism and to suppress serum thyroid-stimulating hormone (TSH) for the treatment of differentiated thyroid cancer (DTC)
An increased LT4 dose was required for 50% of patients in the TSH suppression group for thyroid cancer and 26% of patients in the replacement group for hypothyroidism
Risk factor analysis showed that remaining thyroid gland, the time interval between starting RIF and TSH measurement, and baseline LT4 dose per kg body weight were clinically relevant variables
Summary
Levothyroxine (LT4) is widely used to replace thyroid hormone in patients with hypothyroidism and to suppress serum thyroid-stimulating hormone (TSH) for the treatment of differentiated thyroid cancer (DTC). The main site of deiodination is the liver, several other organs including the kidneys metabolize LT4. LT4 is a substrate for CYP3A4, a member of the hepatic cytochrome P450 family of enzymes. Rifampin (RIF) is used to treat active tuberculosis (TB) and latent TB infection (LTBI), as well as disease caused by nontuberculous mycobacteria (NTM) such as Mycobacterium avium complex. These mycobacterial diseases require long-term RIF treatment. RIF is a potent inducer of hepatic cytochrome P450 enzymes, especially CYP3A4, increasing the metabolism of many drugs that are metabolized by the liver [2,3]. Levothyroxine (LT4) and rifampin (RIF) are sometimes used together; no clinical studies have assessed the effects of these drugs on thyroid function or the need to adjust LT4 dose
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