Drinking a small amount of water before the thyroid function test does not usually have a significant effect on the results, but if you drink a large amount of water, it is recommended that the test be performed at another time. Thyroid function test is a blood test which includes TSH (Thyroid Stimulating Hormone), Total T3 (Total Triiodothyronine), Total T4 (Total Thyroxine), Free T3 (Free Triiodothyronine), Free T4 (Free Thyroxine), TPO Antibody (Thyroid Peroxidase Antibody), TG Antibody (Thyroglobulin Antibody). Thyroid function tests should be performed on an empty stomach after 8-12 hours, but drinking a small amount of water will not significantly affect the results, and you can inform your physician to clarify whether or not it is possible to perform a blood test.