Most commonly, a person does not have to fast before a TSH test according to the Cleveland Clinic. The test can be performed any time of the day.
A TSH test, also known as a thyroid stimulating hormone test, is used to test how the thyroid is working. The thyroid uses iodine from the food a person eats to make hormones that are secreted throughout the body, as stated by WebMD. In some people, the thyroid can work too much, or not enough. When either of these things occur, it can cause the body's metabolism to work too hard or too slow.