ST-1891-201 is a multicenter, randomized, double-blind, partial crossover, Phase 2 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