1.Serum thyroid hormone measurement 100% of serum T4 is secreted from the thyroid gland. Since TT4 is affected by serum thyroid binding proteins, especially thyroid binding globulin (TGB), it is FT4 that really represents the functional status of the thyroid gland, and FT4 in hypothyroid patients is often lower than the normal range. 20% of serum T3 comes from thyroid secretion and 80% from the conversion of T4 in the periphery, so T3 is not a good representative of thyroid function. In hypothyroid patients, under the effect of elevated TSH, the conversion of T4 to T3 increases, and T3 can be low, normal or even elevated, and TT3 and FT3 are of little significance for the diagnosis of hypothyroidism. 2.TSH measurement TSH and thyroid hormone have a very good negative correlation, T4 and FT4 reduction combined with TSH elevation is certain for the diagnosis of primary hypothyroidism; TT4 and FT4 reduction with no TSH elevation should be considered for the possibility of secondary hypothyroidism. The thyroid uptake rate of I131 is not meaningful for the diagnosis of hypothyroidism because the severity of hypothyroidism varies, and the performance of thyroid uptake rate varies between early and late onset of hypothyroidism, and can be low or normal or elevated. 4.TSH test and TRH test The emergence of highly sensitive TSH measurement kits has made the differential diagnosis of primary hypothyroidism and secondary hypothyroidism easy, and TRH test is rarely performed for the differential diagnosis of primary and secondary hypothyroidism at present. Determination of autoantibodies to the thyroid gland is helpful to understand the cause of hypothyroidism, but it is not necessary to diagnose hypothyroidism, because some patients with chronic lymphatic thyroiditis can have negative antibodies and normal thyroid function. 6.Other The lack of thyroid hormone causes cholesterol decomposition