To the Editor.—
Jacobson et al (1980;243:733) and Shroff and Jones (1980;244:658) recently stressed the differences between various thyroxine (T4) preparations. Equally important are differences between preparations and batches of desiccated thyroid and thyroglobulin. The ratio of levothyroxine sodium to liothyronine sodium in the Armour Thyroid (desiccated thyroid) is 4.3; in Proloid (thyroglobulin), 2.9. Both preparations are sometimes more potent than claimed.1 Generic thyroid preparations have variable T4 and triiodothyronine (T3) content (1980;243:549).2 As a result, sometimes the patients treated with such preparations have development of thyrotoxicosis with normal T4 and high T3 levels. We observed this in 28 adults (27 of them women) aged 18 to 70 years. Blood was drawn in the morning, 24 hours after the previous dose; in view of the short half-life of T3 in the blood, this could only underestimate the degree of hypertriiodothyroninemia.