How does diurnal variation affect thyroid-stimulating hormone testing, and what is the recommended timing for sampling?

• A. TSH has diurnal variation with a morning peak; sample in the morning after fasting if possible.
• B. TSH shows diurnal variation with a morning peak; sample in the morning after fasting if possible.
• C. TSH should be drawn only in the afternoon.
• D. TSH should be drawn after exercise.

Answer: (B)

Diurnal variation in TSH means its level changes over the day, with a tendency to be higher in the morning. Because this rhythm can affect the result, the timing of the blood draw is important to get a consistent, comparable measurement. The preferred approach is to sample in the morning, ideally before meals, and after fasting if possible. This standardizes conditions and minimizes the impact of the natural daily fluctuation. Drawing later in the day or after exercise could introduce additional variability and is not as reliable for interpreting TSH.