IMPE Abstracts

A 9 - year-old boy was referred to our hospital because of precocious puberty. At the first visit, thyroid function test was performed using electrochemiluminescent immunoassay (ECLIA) and the serum TSH level was markedly elevated (32.30 μIU/mL), but the serum level of free T4 was within the reference interval. Anti-thyroid antibodies were negative and thyroid ultrasonography was normal. He diagnosed as central precocious puberty, but no clinical symptom of hypothyroidism. After levothyroxine replacement therapy, free T4 was elevated over the normal reference (2.22 ng/dL), but TSH was still elevated (31.00 μIU/mL). One month ago, he had a thyroid function test using radioimmunoassay at another hospital, and TSH level was 0.88 μIU/mL (reference range 0.3-4.1 μIU/mL), which suggested false elevation of the TSH level at our hospital lab. In our hospital, the patient serum was diluted 10-fold and measured as TSH 37.6 μIU/mL (recovery rate 135%). Incubation of the patient’s sera with the sera from two patient known to exhibit hypothyroidism (TSH 58.0 μIU/mL and 7.5 μIU/mL, respectively) at a 1:1 ratio and measuring TSH levels showed decreased recovery (56.0 μIU/mL and 37.9 μIU/mL respectively). After polyethylene glycol precipitation, serum TSH level was 3.57 μIU/mL using ECLIA, which suggested the existence of interfering substances, macro-TSH. Macro-TSH is mainly a complex of TSH with anti-TSH autoantibodies. Due to its large molecular size, it accumulates in the circulation resulting in elevated serum TSH concentrations. The biologically active fraction of TSH is normal, treatment with thyroxine is not necessary. The prevalence of macro-TSH is no more than 1% in adult patients with subclinical hypothyroidism. Although, gel filtration chromatography Best our knowledge, he is the first child in Korea, with falsely elevated TSH concentration. If the patient's clinical presentation and test results are inconsistent, it is essential that the clinician suspect the interference in clinical laboratory tests.