FRI519 Misleading Free T4 Due To Assay Interference

Disclosure: R.M. Pradhan: None. H.K. Rai: None. B. Rai: None. M. Batra: None. Background: Despite Equilibrium Dialysis being the gold standard for measurement of Free t4 (FT4), competitive Immunoassays are the go-to test for clinicians due to its cost and fast turnover. Measuring FT4 which accounts for 0.02% of total T4 is a challenge, but several factors interfering with the assay, sometimes makes it even more difficult. Case Report: 55 years old Male with PMH of CKD III, HTN, HLD, RCC s/p left partial nephrectomy in 2015, and IgG kappa monoclonal gammopathy following with endocrinology for hypothyroidism secondary to Hashimoto’s disease. Patient was doing well on Levothyroxine 125 mcg po daily. He denied any hypothyroidism/ hyperthyroidism symptoms. Patient was not on any heparin and amiodarone. TFT was done for routine follow up. Initial TFT was done via one step automated immunoassay on the Siemens Centaur platform (Center 1): TSH was 1.34 mlU/L (N 0.4-4.5 mlU/L) and Free T4 was 4.9 ng/dL (N 0.8-1.8 ng/dL). As the free T4 was inconsistent to the clinical picture and both the TSH and free T4 were incongruent to each other, it was difficult to interpret. After discussing with the patient, the labs were repeated with total T3 and total T4 as well. Repeat labs in 10 days with same assay (Center 1) showed TSH: 2.34 mlU/L, Free T4: 5.3 ng/dL (elevated), Total T4: 6.3 mcg/ dl (N 4.9-10.5 mcg/ dL) and Total T3: 124 ng/dL (N 76-181 ng/dL). On chart review, patient had similar finding about 2 years ago when the test was done at Center 1 which showed raised free T4 with normal TSH and repeat labs in a different center (Center 2) were normal. Given the h/o monoclonal gammopathy and varied test results in different center, TFTs were repeated in a second diagnostic center. Center 2 used an Abbott’s two chemiluminescent immunoassay (CLIA) which showed that the free T4: 1.01ng/dL (N 0.8-1.8 ng/dL), TSH: 1.39 mcUnit/mL (N 0.4-5 mcUnit/mL), Total T4: 7mcg/dL (N 4.9-11.7 mcg/dL) and Total T3: 85 ng/dL (N 75-165 ng/dL). Patient was recommended to continue with the same dose of levothyroxine and follow up routinely. HAMA and other thyroid autoantibodies were not measured. Conclusion: Various medications and medical conditions that interfere with the binding proteins are sometimes easy to discern by history, but the difference is assays need clinical suspicion from the physicians to avoid further unnecessary labs and radiological investigations. Thyroid hormone autoantibodies/other interfering molecules are likely culprit for our patient’s abnormally high free T4 in the one-step automated immunoassay as the wash out in the two-step CLIA likely resulted in normal values on a clinically euthyroid patient. Presentation: Friday, June 16, 2023