Do hormone laboratory tests require fasting?

Fasting Requirements for Hormone Laboratory Tests

Most hormone tests do NOT require fasting, with important exceptions being insulin, C-peptide, gastrin, and VIP, which require an 8-hour fast for accurate interpretation. 1

Hormones That REQUIRE Fasting (8-Hour Minimum)

Pancreatic/GI Hormones

• Insulin, proinsulin, C-peptide, and insulin/glucose ratio must be measured fasting, particularly when evaluating for insulinoma 2, 1
• For suspected insulinoma, a supervised 48-72 hour fast with serial measurements is the gold standard diagnostic test 1
• Gastrin requires an 8-hour fast when working up suspected gastrinoma 2, 1
• VIP (vasoactive intestinal peptide) requires an 8-hour fast for VIPoma evaluation 2, 1

Metabolic Tests

• Fasting plasma glucose requires minimum 8-hour fast (no caloric intake) for diabetes diagnosis 2
• Oral glucose tolerance test requires 8-14 hours of fasting before glucose load administration 2, 1

Hormones That DO NOT Require Fasting

Thyroid Hormones

• TSH and free T4 do not require fasting 2, 1
• However, TSH shows statistically significant postprandial suppression 3, which can result in reclassification of 75% of subclinical hypothyroid patients when comparing fasting versus non-fasting samples 3
• For consistency in serial TSH monitoring, collect samples at the same time of day to minimize diurnal variation 1
• Never interpret thyroid function during acute illness due to euthyroid sick syndrome 1

Reproductive Hormones

• LH and FSH do not require fasting, though timing during menstrual cycle matters (day 3-6 recommended for women) 1
• Prolactin should be measured as morning resting levels (not postictal), but fasting is not required 1
• Testosterone in women shows higher fasting levels in apparently healthy women 4, but no specific fasting requirement exists in clinical guidelines 2, 1

Adrenal Hormones

• ACTH and cortisol should be measured in early morning (around 8 AM) for diagnostic accuracy based on circadian rhythm, but fasting is not specifically required 1
• Metanephrines (plasma and urine), catecholamines, and dopamine for pheochromocytoma/paraganglioma workup do not require fasting 1

Other Hormones

• Growth hormone/IGF-1 do not have specific fasting requirements in routine clinical practice 1
• Chromogranin A, somatostatin, pancreatic polypeptide, calcitonin, and PTH-related peptide do not require fasting 2, 1

Critical Clinical Pitfalls

Medication Interference

• Proton pump inhibitors (PPIs) cause artifactual elevations in chromogranin A levels, but should not be discontinued if medically necessary 1
• For gastrin measurement, PPIs should ideally be stopped 10-14 days before testing with H2 antagonists substituted and stopped 48 hours before 1
• Never stop PPIs in suspected gastrinoma without medical supervision due to risk of severe complications 1

Dietary Restrictions

• 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, walnuts, and must avoid coffee, alcohol, and smoking 1

Physiologic Considerations

• Fasting causes marked increases in cortisol, ACTH, beta-endorphin, and catecholamines even without hypoglycemia 5
• Fasting decreases TSH and T3 levels while increasing growth hormone 5
• These physiologic responses to fasting can confound interpretation if not anticipated 6, 5