is tsh a fasting lab
It is not fasting itself that is the issue. It is whether it is a good idea to take meds prior ti the blood draw. Taking t4 containging meds before a blood draw will raise the free t4 by about 20% for several hours after swallowing the pill. This is due to a surge of t4 being released from the med. This will not affect TSH or free t3, only the free t4 result. This could result in a free t4 looking higher than it really is and make cause a doctor to think your levels are ok. So I would do as AR said. Fast and have the blood drawn early in the morning so you can take the med immediately after the blood draw. Consistency is very important. TSH keveks are also at their highest in the morning and some cases of mild hypo can be missed if the blood is drawn later in the day when levels begin to drop.