T3 and T4 are indicators of thyroid function checked by blood sampling, and both reflect the state of thyroid hormone synthesis and secretion. Routine thyroid function tests include TSH (thyroid stimulating hormone), T4 (thyroxine), T3 (triiodothyronine), FT4 (free thyroxine) and FT3 (free triiodothyronine). For thyroid function tests, endocrinologists actually focus on the free indicators, namely FT3 and FT4, both of which, combined with TSH, are the main indicators for a clear diagnosis of thyroid disease. Because of the different kits in each hospital, the normal range of each of these indicators varies. Generally, FT3 and FT4 are higher than normal, while TSH decreases, indicating thyrotoxicosis, and further clarification of the cause of thyrotoxicosis is needed. If FT3 and FT4 are decreased along with elevated TSH, the presence of hypothyroidism is considered. Thyroid disorders are one of the most common diseases of the endocrine system. Thyroid hormones are involved in a variety of body metabolisms and therefore can seriously affect health when thyroid function is abnormal. Thyroid function tests have a high detection rate for thyroid disorders, so they are now becoming a routine medical checkup.