T4

About This Test
T4 (thyroxine) is the main hormone produced by the thyroid gland and is converted in the body to the more active T3. It affects energy use, temperature, and growth. Abnormal levels point toward underactive or overactive thyroid and are interpreted with TSH (and sometimes FT4/FT3) for accuracy. This test is commonly included in routine thyroid evaluations to screen for potential problems early.

Patient Preparation
No fasting needed

Best Time to Test
Morning preferred, but can also be done anytime

Tests Included
T4

Why is it done
Work-up of fatigue, weight change, hair loss, menstrual changes, or mood shifts; diagnosis and monitoring of hypothyroidism or hyperthyroidism; dose-adjustment guidance for thyroid hormone replacement; clarification of abnormal TSH or symptoms suggestive of thyroid disease.