To the Editor.—
We have read with interest the article by Slag et al, "Free Thyroxine Levels in Critically Ill Patients" (1981;246:2702), and wish to call attention to several errors of interpretation. In Fig 1, the Damon Diagnostics microcapsule is depicted as showing thyroxine (T4) tracer bound to thyroid-binding globulin (TBG) in the capsule, instead of T4 tracer bound to specific anti-T4 antibody. The selection of patients is presumed to be representative but consists of 94% male and 6% female patients. In the intensive care unit (ICU), patients with a total T4 level below 5 μg/dL, the free T4 index was low in 30% and the T4-TBG ratio was low in 35%. The commercial methods of Clinical Assays and Damon Diagnostics gave normal free T4 results for these patients, consistent with better diagnostic performance of these methods in comparison with existing standard
Buehler RJ, Sullivan WM. Free Thyroxine Levels in Critically III Patients. JAMA. 1982;248(7):828–829. doi:10.1001/jama.1982.03330070016010
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