This study is looking at how safe and effective Armour Thyroid is compared to synthetic T4 for treating adults with primary hypothyroidism. **Primary hypothyroidism** is when the thyroid gland doesn't make enough thyroid hormone. **Armour Thyroid** is a natural form of thyroid hormone, while **synthetic T4** is man-made. The study is for people who have been on synthetic T4 for at least 12 months and have stable thyroid levels (measured by a hormone called TSH). The study will see how well people switch from synthetic T4 to Armour Thyroid.

• Length of Study: Participants need to have been diagnosed for at least one year and on synthetic T4 for at least one year before starting.
• Visits Needed: Screening and dosage monitoring visits are required to ensure safety and effectiveness.
• Potential Risks: Those with allergies to ingredients in Armour Thyroid or who have had certain surgeries may not qualify.