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August 20, 1982

Free Thyroxine Levels in Critically III Patients

JAMA. 1982;248(7):828-829. doi:10.1001/jama.1982.03330070016010

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