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Relationship of FT4 with the log TSH. A) TSH and FT4 performed on the Siemens Immulite 2500 analyzer show a poor correlation. B) Improved correlation of the inverse log-linear relationship between TSH and FT4 when FT4 is measured by ultrafiltration followed by LC-MSMS. Adapted from (18) with permission from the American Association for Clinical Chemistry.
