Do hormone lab tests require fasting?

Fasting Requirements for Hormone Laboratory Tests

Most hormone blood tests do not require fasting, with important exceptions including insulin-related tests (insulin, proinsulin, C-peptide, insulin/glucose ratio) and certain neuroendocrine tumor markers (gastrin, VIP) which require an 8-hour fast. 1

Tests Requiring Fasting (8-Hour Fast)

Neuroendocrine and Pancreatic Hormone Tests

• Gastrin testing requires an 8-hour fast for accurate measurement in suspected gastrinoma workup 1
• VIP (vasoactive intestinal peptide) requires an 8-hour fast when evaluating for VIPoma 1
• Insulin, proinsulin, C-peptide, and insulin/glucose ratio require fasting for proper interpretation, particularly in insulinoma evaluation 1

Glucose-Related Testing

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

Tests That Do NOT Require Fasting

Thyroid Hormones

• TSH and free T4 do not require fasting according to standard clinical practice 3
• However, TSH levels are significantly suppressed postprandially (after eating), which may affect interpretation in borderline cases 4, 5
• For consistency in serial TSH monitoring, collect samples at the same time of day to minimize diurnal variation 3

Pituitary and Reproductive Hormones

• Growth hormone/IGF-1, prolactin, LH/FSH, and ACTH do not have specific fasting requirements in routine clinical practice 1
• These hormones are listed in neuroendocrine tumor guidelines without fasting specifications 1

Other Hormone Tests

• Chromogranin A, somatostatin, pancreatic polypeptide, calcitonin, and PTH-related peptide do not require fasting 1
• Metanephrines (plasma and urine), catecholamines, and dopamine for pheochromocytoma/paraganglioma workup do not require fasting 1

Important Clinical Considerations

Medication and Dietary Interference

• Proton pump inhibitors cause artifactual elevations in chromogranin A levels, but should not be discontinued if medically necessary 1
• 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

Timing Considerations for Specific Hormones

• TSH levels are highest in early morning and decline throughout the day, with further suppression after meals 4, 5
• Cortisol and ACTH show marked increases during fasting states, particularly on the first morning of fasting 6
• Morning testing is preferred for adrenal function assessment when evaluating multiple hormone axes simultaneously 3

Common Pitfalls to Avoid

• Do not interpret thyroid function tests during acute illness as primary thyroid dysfunction; euthyroid sick syndrome can cause misleading results that should be repeated after metabolic stability 3
• In subclinical hypothyroidism diagnosis, recognize that postprandial TSH suppression may reclassify 75% of borderline cases as normal, potentially affecting treatment decisions 4
• For testosterone and sex hormone testing in women with PCOS, fasting lipid profiles are recommended but sex hormones themselves do not require fasting 1