How should I prepare for hormone and metabolic blood work?

Preparation changes results more than most people expect. The same person can produce a testosterone that looks low at 4 pm and normal at 8 am, a fasting insulin that doubles after a late snack, and a TSH that reads falsely low because of a hair and nail supplement. None of these are laboratory errors. They are predictable effects of when and how the sample was taken, and all of them are avoidable.

What actually needs fasting

The ADA Standards of Care define fasting as no caloric intake for at least 8 hours. That applies to fasting glucose, fasting insulin and any calculated index such as HOMA-IR. Water is fine and is helpful, because a well hydrated vein is an easier draw. Lipids are different. The 2016 joint consensus statement from the European Atherosclerosis Society and the European Federation of Clinical Chemistry recommends non-fasting lipid profiles for most patients, with a repeat fasting sample considered when non-fasting triglycerides exceed 440 mg/dL, which is 5 mmol/L.

Why the morning matters

Cortisol and testosterone follow a daily rhythm, so the clock is part of the test. The 2018 Endocrine Society testosterone guideline instructs clinicians to measure total testosterone on two separate mornings while the patient is fasting, because food lowers testosterone and levels fall through the day. Cortisol is highest shortly after waking, which is why a morning draw is the one that can be interpreted. Prolactin is usually drawn three to four hours after waking, since it falls during the day and rises with sleep.

Supplements that change the result

Biotin is the important one. Many immunoassays use biotin and streptavidin chemistry, so high dose biotin can produce falsely high or falsely low results. The NIH Office of Dietary Supplements records that a single 10 mg dose of biotin has interfered with thyroid function tests taken within 24 hours, and lists interference with thyroid tests, 25-hydroxyvitamin D and troponin, including one case where a falsely low troponin preceded a patient death. Doses in hair, skin and nail products are frequently 5 to 10 mg.

Practical version: book the first appointment of the morning, drink water, skip the biotin containing supplement that day, keep alcohol and heavy lifting out of the day before, and take thyroid medication at the same time relative to the draw each time so results stay comparable. Tell the phlebotomist what you took and when. That single sentence prevents most repeat testing.