TSH Blood Test: Fasting Requirements and Optimal Timing

Fasting is not required for TSH (thyroid-stimulating hormone) testing. TSH is not significantly affected by food intake.

Diurnal Variation and Optimal Timing

• TSH follows a circadian rhythm: peaks between 2–4 AM, nadir in late afternoon (4–8 PM).
• Amplitude: ~50% higher at peak vs. nadir.
• Standard practice: Draw blood in the morning (8–10 AM) for consistency and to capture near-peak levels, maximizing sensitivity for detecting subclinical hypothyroidism.
• If monitoring serial TSH, draw at the same time of day each time.

Other Factors Influencing TSH

• Biotin supplements: High-dose biotin (>5 mg/day) interferes with many immunoassays → falsely low TSH, falsely high free T4/T3. Stop biotin ≥72 hours before draw.
• Medications: Dopamine, glucocorticoids, somatostatin analogs lower TSH; amiodarone, lithium, iodine excess raise TSH.
• Acute illness: Non-thyroidal illness syndrome (low T3, normal/low TSH) – avoid testing during acute hospitalization unless thyroid dysfunction suspected.
• Pregnancy: Trimester-specific reference ranges (lower TSH in first trimester due to hCG cross-reactivity).

Practical Recommendations

1. No fasting needed.
2. Schedule morning draw (8–10 AM).
3. Hold high-dose biotin 72 hours prior.
4. Inform lab of medications, pregnancy, recent illness.
5. Repeat abnormal results in 4–6 weeks before clinical decisions (unless overtly symptomatic).