Study to Assess the Effect of Hepatic Impairment on the Pharmacokinetics of CTP-543

Inclusion Criteria: * Adult males or females aged 18-75 * Body mass index (BMI) ≥ 18.0 and ≤ 42.0 kg/m2 at the time of screening * If of reproductive age, willing and able to use a medically highly effective form of birth control 30 days prior to first dose, during the study and for 30 days following last dose of study medication * Capable of giving informed consent and complying with study procedures Additional Inclusion Criteria for Subjects with Hepatic Impairment: * For moderate hepatic impairment, the subject must have a Child-Pugh score of 7 to 9 at the time of screening. For mild hepatic impairment, the subject must have a Child- Pugh score of 5 to 6 at the time of screening. * No clinically significant change in disease status within the last 30 days before screening * The subject must have a condition consistent with hepatic impairment and associated symptoms, but otherwise be determined to be healthy in the opinion of the Investigator * If diabetic, the subject must have the disease controlled Exclusion Criteria: * History of any clinically significant medical condition, psychiatric disease, social condition, or illness that might confound the results of the study or poses an additional risk to the subject by their participation in the study * Known history of any GI surgery or any condition possibly affecting drug absorption * History of prolonged QT syndrome or a QTc interval with Fridericia's correction (QTcF) \> 470 msec for males or QTcF \> 480 msec for females at Screening visit. * Females who are nursing or pregnant prior to drug administration * Positive for human immunodeficiency virus (HIV) * Positive results for coronavirus infection (COVID-19) at screening or check-in * Positive drugs of abuse or alcohol results at screening or check in (Day -1) Additional Exclusion Criteria for Subjects with Hepatic Impairment: * History or current diagnosis of uncontrolled or significant cardiac disease * Gilbert's syndrome, liver transplant, Wilson's disease, autoimmune liver disease, esophageal variceal bleeding within 3 months prior to screening * Previous diagnosis of hepatocellular carcinoma * Acute or exacerbating hepatitis, fluctuating or rapidly deteriorating hepatic function