Primary hypothyroidism is when your thyroid, a small gland in your neck, doesn't make enough hormones. Some people take a medicine called synthetic T4 to help. This study is looking at a different treatment called Armour Thyroid. We want to see if Armour Thyroid is safe and works as well as synthetic T4. People in the study need to have been on synthetic T4 for at least a year and should have stable thyroid levels. During the study, we will switch from synthetic T4 to Armour Thyroid to see how well the new treatment works. If you have allergies to thyroid medicines or have had alcohol or substance abuse problems, you cannot join.

• The study involves switching medications and monitoring your thyroid levels closely.
• Participants must have used synthetic T4 continuously for at least 12 months before the study.
• People with allergies to thyroid medications or pork cannot participate.