The article entitled "Unusual Events Preceding Hyperthyroidism With Diffuse Goiter" by Bendezu et al in this issue (p 1023) clearly documents two unique presentations of hyperthyroidism. Recent publications1-8 indicate that the spectrum of unusual causes has broadened even since the in-depth review of this subject by Hamilton and Maloof.9 With this thought in mind, I would like to present a schema that I have found practical in approaching this complex subject.
One's clinical impression of hyperthyroidism should be confirmed by an elevated free thyroxine (F T3) index derived by multiplying the serum thyroxine (T4) value by the triiodothyroxine resin uptake (T3U). Calculating an F T4 index generally corrects for changes in thyroid-binding globulin related to certain drugs and chronic illnesses. Rarely, thyrotoxic patient will have a normal F T4 index. For these cases, an elevated serum triiodothyroxine (T3) level, now readily determinable,
Steven G. Dorfman. HyperthyroidismUsual and Unusual Causes. Arch Intern Med. 1977;137(8):995–996. doi:10.1001/archinte.1977.03630200005005