Which hormone tests require fasting?

Hormone Tests Requiring Fasting

The hormone tests that require fasting include insulin/C-peptide/proinsulin, gastrin, VIP, and glucose/insulin ratio, while most other hormone tests including thyroid function, prolactin, LH/FSH, and ACTH do not require fasting. 1

Tests That REQUIRE Fasting (8-Hour Minimum)

Pancreatic Hormone Testing

• Insulin, C-peptide, and proinsulin require fasting for accurate interpretation, particularly when evaluating for insulinoma 1, 2
• The glucose/insulin ratio requires fasting morning levels to assess insulin sensitivity, especially in PCOS evaluation 2
• A supervised 48-72 hour fast is the gold standard for diagnosing insulinoma, with serial measurements of insulin, C-peptide, and glucose 2, 3

Gastrointestinal Hormone Testing

• Gastrin testing requires an 8-hour fast for accurate measurement when evaluating suspected gastrinoma 1, 2
• VIP (vasoactive intestinal peptide) requires an 8-hour fast when evaluating for VIPoma 1

Important Caveat for Gastrin Testing

• Proton pump inhibitors (PPIs) should ideally be stopped 10-14 days before gastrin measurement, with H2 antagonists substituted and stopped 48 hours before testing 2
• However, patients with suspected gastrinoma should never stop PPIs without medical supervision due to risk of severe complications 2

Tests That DO NOT Require Fasting

Thyroid Function

• TSH and free T4 do not require fasting 1
• For consistency in serial monitoring, collect samples at the same time of day to minimize diurnal variation 1
• Do not interpret thyroid tests during acute illness, as euthyroid sick syndrome can cause misleading results 1

Pituitary Hormones

• Prolactin should be measured as morning resting levels (not postictal), but fasting is not required 2
• LH and FSH do not require fasting, though timing during menstrual cycle matters (day 3-6 recommended for women) 2, 1
• ACTH and cortisol should be measured in early morning (around 8 AM) for diagnostic accuracy, but fasting is not specifically required 2, 1

Reproductive Hormones

• Testosterone measurement in women (day 3-6 of cycle) does not require fasting 2
• Research shows fasting may slightly elevate testosterone in healthy women, but this is not clinically significant for diagnostic purposes 4
• Progesterone (mid-luteal phase) does not require fasting 2
• Estradiol does not require fasting 2

Tumor Markers

• Chromogranin A does not require fasting 1
• However, PPIs cause artifactual elevations and should be discontinued if medically safe 1, 3
• Pancreatic polypeptide, somatostatin, and calcitonin do not require fasting 1

Adrenal Testing

• Metanephrines (plasma and urine), catecholamines, and dopamine for pheochromocytoma workup do not require fasting 1

Special Considerations

Timing Recommendations

• Morning testing (around 8 AM) is preferred for adrenal function assessment when evaluating multiple hormone axes simultaneously 2, 1
• This timing recommendation is based on circadian rhythm, not fasting status 2

Dietary Restrictions for Urine Testing

• For 24-hour urine 5-HIAA testing, patients must avoid specific foods for 48 hours including avocados, bananas, cantaloupe, eggplant, pineapples, plums, tomatoes, hickory nuts, plantains, kiwi, dates, grapefruit, honeydew, and walnuts 1
• Coffee, alcohol, and smoking must also be avoided during this period 1

Common Pitfall

• Do not confuse the need for morning timing (for circadian hormones like cortisol and ACTH) with the need for fasting 2, 1
• These are separate requirements: morning timing captures peak hormone levels, while fasting eliminates food-related interference 2, 1