Quotes from McAninch & Bianco’s history of therapy for hypothyroidism: 2016 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4980994/

“In 1965, approximately 4 of every 5 prescriptions for thyroid hormone were for natural thyroid preparations”

“Physicians hesitated to use L-thyroxine monotherapy over concern that it could result in a relative T3 deficiency”