In clinical practice, it is often necessary to test the five items of thyroid function; so what are the five items of thyroid function? The five tests are TT3, TT4, FT3, FT4 and TSH, which are commonly used to evaluate thyroid function. The specific meaning of the five tests: TT3 serum total triiodothyronine, FT3 free triiodothyronine; TT4 serum total thyroxine, FT4 serum free thyroid hormone; TSH thyrotropin. TSH promotes thyroxine release; thyroxine is a tetraiodothyronine containing tetraiodine, which binds to thyroid binding protein (TBG) to form T4 and free to FT4, both of which are interconvertible and sum to TT4. TSH stimulates T4 secretion, and T4 can negatively feedback to inhibit TSH release. t4 is deiodinated to T3, and T3 binds to TBG to form bound T3 and exists free to form FT3. bound TSH stimulates T3 secretion and T3 can inhibit TSH release by negative feedback. The current view of the significance of the five tests: The five tests are a basic topic for the detection of nail function and have their importance and controversy. The current mainstream view is to use FT3 and FT4 as diagnostic criteria for the following reasons: While TT3 and TT4 are total thyroxine levels, which are affected by many factors, especially the thyroid binding protein TBG, FT3 and FT4 are free thyroxine, which has relatively little influence. FT4 and FT3 are sufficient. Summary of the significance of total thyroid hormone and free thyroid hormone measurement in Practical Internal Medicine (14th edition): 1. TT3 measurement is the most sensitive indicator for the diagnosis of hyperthyroidism in hyperthyroidism, and serum TT3 can be four times higher than normal, making it easier to distinguish hyperthyroidism from normal people. However, this measurement is easily affected by changes in blood TBG concentration, so more attention should be paid when determining the results. For example, if TBG increases during pregnancy, TT3 may also increase. This is particularly important when determining the functional status of the thyroid gland in patients with hyperthyroidism in pregnancy and when deciding on the dosage of medication. 2. FT3 is very sensitive for the diagnosis of hyperthyroidism but the presence of autologous thyroid antibodies can interfere with FT3 and FT4 results: (1) FT4 will be elevated when endogenous T4 antibodies are present in the blood; (2) FT3 will be elevated when endogenous T3 antibodies are present. In this case, a TRH excitation test, for example, is needed to help further determine. It can be seen that both TT3, TT4 and FT3, FT4 are affected. The “total” theory or the “free” theory should be combined with clinically relevant indicators. The significance of the five tests: 1, considering the effect of TBG on TT4 and TT3 in the serum, it is recommended to add TBG test to facilitate comprehensive analysis to determine whether TT4 and TT3 elevation is pseudo; 2, clinical consideration can also be given to the addition of thyroid stimulating antibody TRAB to assist in the judgment; 3, thyroid-related disease diagnosis should not rely solely on test data, the most important thing is to ask detailed The diagnosis of thyroid-related diseases should not rely solely on test data, but most importantly on detailed medical history, physical examination and relevant clinical symptoms; 4. TT4 is currently an extremely critical indicator for responding to disease and adjusting medication. When TT4 drops to the normal range or drops significantly, drug reduction is more reliable, while FT4 is prone to relapse or fluctuation even if it drops. 5, in order to comprehensively assess thyroid function, it is recommended that the five indicators of thyroid function should still be tested together.