When we commonly evaluate the thyroid function, we measure TSH, free T3 and free T4. However, there are some cases that are difficult to diagnose because of the existence of thyroid hormone autoantibodies. We experienced a case of a 14-year-old girl with diffuse struma whose TSH, free T3 and free T4 were elevated by detected with Amerlex M free T3 and free T4 kits, although she did not have any symptoms. As the free T4 level was low by the equilibrium dialysis method, we diagnosed this case as hypothyroidism due to chronic thyroiditis. Because we found thyroid hormone antibodies in her serum, we thought that this case presented pseudoelevations of free T3 and free T4 by the analogue method.