Coadministration has not been studied. Levothyroxine is metabolized by deiodination (by enzymes of deiodinase family) and glucuronidation. Cases of hypothyroidism have been reported when combining levothyroxine with ritonavir, indinavir or lopinavir/ritonavir. This interaction has been attributed to the induction of glucuronidation thereby increasing elimination of levothyroxine. Close monitoring of thyroid hormone parameters is recommended and adjustment of the levothyroxine dose may be necessary if clinically indicated. Monitoring of thyroid-stimulating hormone (TSH) is recommended for at least the first month after starting and ending ritonavir-containing treatment regimens.