It is ground up porcine thyroid. When it was first produced, the medication was not well standardized and its effects were unreliable without predictable responses. It is likely better standardized now.

The American Thyroid Association and the American Association of Clinical Endocrinologists do not recommend it in their guidelines.

Desiccated thyroid hormone should not be used in pregnancy as it has a higher percentage of T3 than T4. Since it is the T4 that predominantly crosses the placenta, desiccated thyroid hormone can underdose the fetus.

A small randomized double-blind crossover study published in 2013 compared the use of desiccated thyroid extract (DTE) with levothyroxine for the treatment of hypothyroidism.

70 patients with primary hypothyroidism who were on a stable dose of levothyroxine for 6 months were randomized to either DTE or levothyroxine for 4 months. Then they were all switched over to the other one for 4 more months.

The authors found that there was no difference in symptoms or quality of life between the two groups. However, half of the patients preferred DTE and only about 20% preferred levothyroxine.

The study also found that 15% of patients on DTE had hyperthyroid symptoms. They also had about 3 pounds greater weight loss.