title: Probable interaction between levothyroxine and ritonavir: Case report and literature review.
abstract: OBJECTIVE
  A case of probable interaction of levothyroxine and ritonavir is presented along with a review of the relevant literature and recommendations on clinical management.
  
  
  CONCLUSIONS
  A 37-year-old woman with human immunodeficiency virus infection who had recently undergone thyroidectomy for a benign multinodular goiter presented to a clinic with hypothyroidism (she was also found to be pregnant), and treatment with levothyroxine 75 μg daily was initiated. While receiving antiretroviral therapy (abacavir-lamivudine and lopinavir-ritonavir) during pregnancy, the patient had persistently elevated serum thyroid-stimulating hormone (TSH) concentrations (up to 125.89 μIU/mL) despite gradual escalation of the levothyroxine dosage to 175 μg daily. An interaction between ritonavir and levothyroxine was suspected, and dolutegravir was substituted for lopinavir-ritonavir. Within 4 months, the TSH concentration had normalized. The daily levothyroxine dose was tapered over a 5-month period and stabilized at 125 μg, and TSH concentrations remained within the normal range over an 18-month follow-up period. Scoring of the case using the Drug Interaction Probability Scale yielded a score of 6 out of 11, indicating a probable interaction between levothyroxine and ritonavir. A literature search identified 4 reported cases of interactions involving levothyroxine and ritonavir.
  
  
  CONCLUSIONS
  A potentially serious and underrecognized drug interaction between ritonavir and levothyroxine was observed in a pregnant woman with postthyroidectomy-related hypothyroidism. This case and a review of other cases reported in the literature indicate that higher-than-usual doses of levothyroxine may be required in patients who are taking ritonavir concurrently.
authors.affiliation: Rocky View General Hospital, Calgary, Alberta, Canada.
authors.forename: Rahul
authors.initials: R
authors.lastname: Sahajpal
authors.affiliation: Department of Medicine, University of Alberta, Edmonton, Canada.
authors.forename: Rabia A
authors.initials: RA
authors.lastname: Ahmed
authors.affiliation: Faculty of Pharmacy & Pharmaceutical Sciences, University of Alberta, Edmonton, Canada.
authors.forename: Christine A
authors.initials: CA
authors.lastname: Hughes
authors.affiliation: Northern Alberta Program, Royal Alexandra Hospital, Edmonton, Canada. michelle.foisy@albertahealthservices.ca.
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Foisy
mesh_terms: D000328:Adult; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D015658:HIV Infections / Q000188:drug therapy*; D017320:HIV Protease Inhibitors / Q000494:pharmacology* / Q000627:therapeutic use; D006801:Humans; D007037:Hypothyroidism / Q000188:drug therapy*; D019438:Ritonavir / Q000494:pharmacology* / Q000627:therapeutic use; D013974:Thyroxine / Q000494:pharmacology* / Q000627:therapeutic use
chemical_list: D017320:HIV Protease Inhibitors; D019438:Ritonavir; D013974:Thyroxine
keywords: drug interaction; levothyroxine; ritonavir