In the early 1960s, desiccated thyroid hormones (thyroid extract) began to be replaced by  levothyroxine (synthetic T4), or by combinations of T4 and T3. Replacement occurred faster in the United Kingdom than in North America, but by the 1980s more patients were being prescribed synthetic T4 (levothyroxine) or synthetic T4/T3 combinations than desiccated thyroid extract.Several reasons have been identified as to why prescriptions changed from desiccated thyroid treatment. Although thyroid extract was useful and usually effective, some patients continued to complain of fatigue, weight gain, or other symptoms. Dosing until the 1960s was often a matter of prolonged adjustment trials. It was known that not all of the iodine content of thyroid extract was in the form of effective T4 and T3 and that actual content of available preparations varied more than the permitted 15%. It was hoped that better dosing precision with levothyroxine (synthetic) alone would increase the proportion of patients effectively treated. In 1980, a widely publicized investigation published in JAMA revealed continued large ranges of hormone content and potency in all of the available thyroid extracts on the American market. By the 1960s, it was known that thyroxine was the essential hormone produced by the thyroid gland, and that most T3 was manufactured in other parts of the body by deiodination of thyroxine. It was demonstrated in hypothyroid animals and people that replacement of thyroxine alone corrected the measurable manifestations (laboratory test results) of hypothyroidism.
Before modern assays, the potency was specified only by iodine content ("not less than 0.17% and not more than 0.23%"), rather than hormonal content or activity. History
The earliest oral treatment for hypothyroidism consisted of thyroid extract. George Redmayne Murray of the United Kingdom first described treatment of myxedema with thyroid extract in 1891, and published a description of long-term successful treatment (28 years) of a patient with myxedema (severe hypothyroidism) in 1920 His treatment was quickly adopted in North America and Europe. The first recorded American use dates to 1891 by a woman who was still taking it 52 years later at 84 years of age Desiccated thyroid extract is prepared from pig thyroid glands. The glands are dried (desiccated), ground to powder, combined with binder chemicals, and pressed into pills. This was a new use for parts that were previously unwanted slaughterhouse offal, and Armour and Company, the dominant American meatpacker in the 20th century, supplied the best-known brand of thyroid extract.Replacement by thyroid extract in hypothyroidism was one of the most effective treatments of any disease available to physicians before the middle of the 20th century, and in severe cases afforded dramatic relief of the myriad symptoms. The decision to treat was usually based on the presence of signs and symptoms of hypothyroidism because there were no accurate, readily available laboratory tests of thyroid function. Many less severe cases of hypothyroidism went untreated. Dosage was regulated by improvement of symptoms. Desiccated Thyroid became a commercial treatment option in 1934 with Westhroid,.