Traditionally hypothyroidism is treated with a thyroxine supplement using prescriptions like levothyroxine.  This therapy only supplies T4 hormone, the inactive form of thyroid hormone that must be converted in the body to T3 (triiodothyronine). Using compounding and desiccated thyroid, supplement therapy is able to provide both the T4 and T3 forms which provides greater benefit to the patient.

Effects of thyroxine as compared with thyroxine plus triiodothyronine in patients with hypothyroidism.
https://www.ncbi.nlm.nih.gov/pubmed?term=9971866 

Levothyroxine therapy and serum free thyroxine and free triiodothyronine concentrations.
https://www.ncbi.nlm.nih.gov/pubmed?term=11929079