Wikipedia
Liotrix is a 4:1 mixture of thyroxine (T) and triiodothyronine (T) made synthetically. It is used to replenish thyroid hormones in thyroid deficiency and hypothyroidism. The only brand of liotrix available in the U.S. is Thyrolar, manufactured by Forest Laboratories.
As of 2013, Forrest Laboratories has released the following statement regarding the limited availability of Liotrix:" U.S. Pharmacopeia, an official public standards-setting authority for prescription and over-the-counter medicines and other health care products manufactured or sold in the United States, has mandated new specifications for a component used in the manufacturing of Thyrolar. As a result, all strengths of Thyrolar are currently on long-term back order while modifications necessary to meet these new specifications are made."