A Study Evaluating the Safety and Efficacy of Hormone Replacement Therapy With ST-1891 Compared to Levothyroxine in Patients With Primary Hypothyroidism

Inclusion Criteria: * Diagnosed with primary hypothyroidism * On continuous thyroid replacement therapy with levothyroxine for at least 12 months immediately prior to Screening * On a stable daily dose of levothyroxine for the 3 months prior to Screening * Willing to give written informed consent for the Study Exclusion Criteria: * Any clinical condition or previous surgery that might affect the absorption, distribution, biotransformation or excretion of ST-1891 or levothyroxine * Female patients who are pregnant or are breastfeeding starting 30 days prior to Screening * Anticipated initiation or change in concomitant medications * Concomitant use of prohibited medications * Currently participating in another clinical study or have received active treatment with an investigational drug within 30 days or 5 half-lives of the investigational drug of Screening, whichever is longer