A Study of Armour® Thyroid Compared to Synthetic T4 (Levothyroxine) in Previously Hypothyroid Participants

* Participants who have a diagnosis of primary hypothyroidism made ≥ 12 months before study entry (Visit 1). * Be on continuous thyroid replacement therapy with synthetic T4 for primary hypothyroidism for at least 12 months immediately prior to the Screening Visit (Visit 1). * Be on a stable Food and Drug Administration (FDA)-approved daily dose of synthetic T4 for a minimum of 3 months prior to the Screening Visit (Visit 1). Must enter the study on the same stable dose. * Have euthyroid status indicated by at least 1 documented TSH value within normal reference range (0.45 - 4.12 mIU/L, inclusive) at a minimum of 6 weeks and maximum of 12 months prior to the Screening Visit (Visit 1). Also have a confirmed TSH value within the normal reference range drawn at the Screening Visit (Visit 1). * Male and female participants willing to minimize the risk of inducing pregnancy for the duration of the clinical study and follow-up period (35 days after last dose of study intervention).

* Any clinical condition or previous surgery that might affect the absorption, distribution, biotransformation, or excretion of Armour Thyroid or synthetic T4. * History of alcohol or other substance abuse within the previous 5 years. * Known or suspected allergy or intolerance to any ingredients of Armour Thyroid, including its excipients, levothyroxine (T4), other thyroid replacement medications, or pork products. * Have received active treatment with an investigational drug within 30 days or 5 half lives, whichever is longer, of Screening Visit (Visit 1). * Current enrollment in an investigational drug or device study or participation in such a study within 60 days of entry into this study.