The serum concentrations of thyrotropin (TSH) were measured by means of radioimmunoassay, in 98 cases of normal controls, 51 cases of hyperthyroidism, 80 cases of primary hypothyroidism and 4 cases of secondary hypothyroidism to evaluate the diagnostic significance in various functional states of the thyroid. The obtained data were analyzed in correlation with other thyroid function test values in various phases of the functional thyroid diseases. The results were as follows: 1) The serum TSH concentration in normal control group was $<1.3{sim}8.0{mu}U/ml$. 2) The measurement of serum TSH was more significant in diagnostic accuracy compared with that of serum $T_4(75.0{pm}12.2%)$. Free $T_4$ Index ($64.2{pm}15.2%$), serum $T_3(41.0{pm}21.0%);or;T_3$ resin uptake ($41.1{pm}15.8%$) in evaluation of primary hypothyroidism. 3) In case of overt hypothyroidism, the serum TSH and $T_4$ were both abnormal, compatible with the clinical diagnosis, while in case of preclinical or mild hypothyroidism, the serum $T_4(41.2{pm}23.8%);or;50.0{pm}25.0%)$ was much less reliable than serum TSH. 4) In the treatment of primary hypothyroidism with desiccated thyroid, the administration of 1 grain of the hormone per day was sufficient to suppress the serum concentration of TSH to normal range. It showed that the measurement of serum TSH concentration was a significant criteria in evaluating the efficiency of the treatment of hypothyroidism. 5) The measurement of serum TSH concentration is a very significant method in the early detection of hypothyroidism induced during or after the treatment of the hyperthyroidism with antithyroid drugs or radioactive Iodine ($^{131}I$).