PSAT329 A Delayed Diagnosis of Subclinical Hypothyroidism Due to Interferences with Thyroid Function Immunoassays

INTRODUCTION: Automated immunoassays used to evaluate thyroid function tests (TFTs) are susceptible to various forms of interference that can lead to inaccurate results and misdiagnosis. CASE DESCRIPTION: A 55-year-old male was diagnosed with abnormal TFTs since 2017 [TSH 7.32 mIU/L (range 0.4-4.5), FT3 5.1 pg/mL (range 2.3-4.2), and FT4 1.2 ng/dL (range 0.9-2.2)]. Thyroid peroxidase antibody and thyroglobulin antibody were positive. He was on levothyroxine 50 mcg daily since 2017, and he did not feel significantly different despite treatment. He was referred to endocrine clinic in 2021. He had no symptoms of hypothyroidism or hyperthyroidism except occasional palpitations and headache. He reported blurred vision due to cataracts. He denied thyroid diseases in first degree relatives. He denied taking biotin or desiccated thyroid extract. Physical examination was unremarkable. Differentials included TSH-producing adenoma, assay interference, exogenous T3 use, T3 thyrotoxicosis with interference of TSH assay, and thyroid hormone resistance. Levothyroxine was discontinued in May 2021. Other pituitary hormones including alpha-subunit and MRI pituitary were unremarkable, which ruled out TSH-producing adenoma. Human anti mouse antibody (HAMA) was normal. In June 2021, prior to seeing genetics, FT4, FT3 and Total T3 were checked with equilibrium dialysis method and values were 1.2 ng/dL (range 0.9-2.2), 238 pg/dL (range 210-440) and 105 ng/dL (range 76-181), respectively and TSH 9.13 mIU/L (range 0.40-4.50), thyroglobulin 0.6 ng/ml (range 2.8-40.9), reverse T3 LC/MS/MS 12 ng/dL (range 8-25). Labs were therefore consistent with subclinical hypothyroidism, and he was started back on levothyroxine 75 mcg every morning. DISCUSSION: TFTs are usually measured by automated immunoassays, and this will have accurate results in most cases. However, immunoassays are vulnerable to interferences that can affect clinical decisions. It is important to consider assay interference such as this and to confirm with equilibrium dialysis method before pursuing further aggressive workup. CONCLUSION: Potential interferences in TFTs should always be suspected whenever discrepancies arise. Awareness of these conditions is critical to the institution of appropriate therapy and preventing misdiagnosis. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.