The serum thyroxine levels were measured by method of Tetrasorb Kit in 69 subjects including 13 subjects in euthyroid state, 31 with hyperthyroidism, 5 with hypothyroidism, 13 with nontoxic diffuse goiter, and 7 with nontoxic nodular goiter. Three parameters of the thyroid function test including thyroxine (T4) levels, 131I uptake (24 hrs) values and PB131I conversion ratio were correlated with clinical manifestations and courses of the disease. 1) The serum T4 levels in the normal subjects were in range of 6.0μg/dl to 14.4μg/dl. (The mean 9.4μg/dl). 2) The diagnostic compatibility of the serum T4 was 93.5% in hyperthyroidism, 100% both in hypothyroidism and in nontoxic diffuse goiter, 86% in nontoxic nodular goiter, or 95.8% in the entire series (Table 1). 3) The diagnosis compatibility of 131I uptake (24 hrs) values and PB131I conversion ratio were less than the serum T4 levels as summarized in Tables 2 & 3. The serum T4 determination by Tetrasorb Kit is a simple, accurate and very useful test of the thyroid function.