Levothyroxine and FDA bioequivalence ratings

A Clinical Inquiry published in the November issue of the journal (“How should thyroid replacement be initiated?,” J Fam Pract 2004; 53:925) discussed initiation of thyroid hormone replacement therapy. The authors and commentator do not specifically state that levothyroxine sodium preparations “usually are administered orally on an empty stomach, preferably one-half to one hour before breakfast or the first food of the day.”¹

Additionally, there is information about the Food and Drug Administration’s (FDA) bioequivalence ratings that became available after the authors had submitted. The October 2004 Supplement to the FDA “Orange Book” now lists generic equivalents to each brand-name levothyroxine product.²

FDA provides the following explanation and chart of therapeutic equivalence evaluations for levothyroxine sodium drug products:

“Levothyroxine Sodium (Mylan) tablets have been determined to be therapeutically equivalent to corresponding strengths of Unithroid (Jerome Stevens) tablets.

“Levo-T (Alara) and Levothyroxine Sodium (Mylan) tablets have been determined to be therapeutically equivalent to corresponding strengths of Synthroid (Abbott) tablets.

“Levo-T (Alara), Unithroid (Jerome Stevens) and Levothyroxine Sodium (Mylan) tablets have been determined to be therapeutically equivalent to corresponding strengths of Levoxyl (King/Jones Pharma) tablets.”

Novothyrox (Genpharm), Thyro-Tabs (Lloyd), and Levolet (Vintage) are listed as requiring further investigation to establish therapeutic equivalence to other levothyroxine products.

Except for these latter 3, if one believes that equivalents of equivalents are equivalent, as illustrated in the Table, all the major levothyroxine preparations appear equivalent.

TABLE
FDA-rated equivalences for the 3 brand name products

Gary N. Fox, MD, Mercy Health Partners Family Practice
Residency, Toledo, Ohio