Should I fast before a thyroid test?

Many labs in India tell people to come fasting for every blood test. For a thyroid test, fasting is not strictly required. But how and when you give the sample can make a small difference, and small differences matter when your TSH is borderline.

Does food or time of day change TSH?

Yes, a little. TSH follows a daily rhythm. It rises at night, is highest in the early morning, and falls through the day. A study that tested the same people on different mornings found TSH was lower when the second sample was taken a couple of hours later, and lower still after breakfast, compared with an early fasting sample.

For most people these changes are small. But if your TSH is close to the upper limit, an early fasting sample might read just above the range on one day and a late-morning sample after breakfast might read normal on another.

So what should I do?

The simple rule is to keep it consistent:

• give the sample at a similar time each time, ideally in the morning
• in a similar state each time, either fasting or after the same light breakfast
• if you also need a fasting sugar or cholesterol test, doing them together in the morning is convenient

What about my thyroid tablet?

If you take levothyroxine, taking it shortly before the blood test can make FT4 read higher for a few hours. Many doctors prefer the sample to be given before that morning’s tablet. Ask your doctor which they prefer, and then follow the same routine every time.

What about supplements?

High-dose biotin, found in many hair, skin and nail supplements, can make a normal thyroid look overactive in many lab methods. Tell your doctor about any supplement, and ask how many days before the test it should be paused.

To understand each line of your report, read the thyroid function test explained. If your TSH is just above the range, see high TSH with normal T4, or check the pattern with the thyroid report reader.