The clinical utility of free thyroxine in oral levothyroxine absorption testing

Original absorption studies for levothyroxine (LT4) were validated using total thyroxine (TT4) measurements. Free thyroxine (FT4) has largely supplanted TT4 in clinical practice. The objective of our study was to assess the clinical utility of FT4 in oral LT4 absorption testing.

In this retrospective electronic health record analysis, we recorded data of patients who underwent LT4 oral absorption testing between November 2010 and January 2012 because of persistent hypothyroidism despite a greater than anticipated weight-based dose of LT4. Patients included had primary hypothyroidism and an absorption test with assessment of both TT4 and FT4 measured at times 0, 30, 60, 90, 120, 180, 240, 300, and 360 minutes. The test was conducted with 1 mg (five 200-μg tablets) of Synthroid® after an overnight fast by a standard nonisotopic method.

A total of 10 patients (3 men/7 women) underwent absorption testing. Prior to testing, the median daily LT4 dose was 250 μg (range, 150 to 350 μg). Three patients were also on liothyronine (10, 20, or 50 μg daily). Based on the calculated amount absorbed, 1 patient demonstrated subnormal absorption, and 9 patients were normal. Median body mass index was 33 kg/m2 (range, 21 to 50 kg/m2). Median calculated absorption was 105% (range, 3.7 to 195.6%). The correlation comparing FT4 and TT4 was 0.88 (95% confidence interval, 0.56 to 0.97; P<.001), a significant correlation.

FT4 and TT4 correlated highly, even in patients who were severely hypothyroid; FT4 may be used interchangeably with TT4 as a qualitative assessment of suspected malabsorption using an oral LT4 absorption test.