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 ($T_4$) levels, $^{131}I$ uptake (24 hrs) values and $PB^{131}I$ conversion ratio were correlated with clinical manifestations and courses of the disease. 1. The serum $T_4$ levels in the normal subjects were in range of $6.0{mu}g/dl;to;14.4{mu}g/dl$. (The mean $9.4{mu}g/dl$). 2. The diagnostic compatibility of the serum $T_4$ 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 diagnostic compatibility of $^{131}I$ uptake (24 hrs) values and $PB^{131}I$ conversion ratio were less than the serum $T_4$ levels as summarized in Tables 2 & 3. The serum $T_4$ determination by Tetrasorb Kit is a simple, accurate and very useful test of the thyroid function.