Before your blood test
Does the time of day change blood test results?
Short answer
For several hormones, yes — a lot. Cortisol peaks in the early morning and falls through the day; testosterone is highest before 10 a.m.; iron and TSH also drift across the day. Draw cortisol within an hour of waking (7–9 a.m.), testosterone between 7 and 10 a.m., and repeat tests at the same time you first tested.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| Cortisol | raises or lowers | Highest 6–8 a.m., lowest around midnight | A.M. cortisol at 7–9 a.m.; P.M. cortisol as the lab specifies [source] |
| Testosterone (total and free) | raises or lowers | Peaks early morning; falls by afternoon, especially in younger men | Draw 7–10 a.m.; confirm a low result with a second morning draw [source] |
| TSH | raises or lowers | Highest late night/early morning; lower in the afternoon | Keep the draw time consistent between tests [source] |
| Iron (serum) | raises or lowers | Higher in the morning | Fasting morning draw [source] |
| Prolactin | raises | Higher after waking and after meals, stress or exercise | Draw 2–3 hours after waking, rested, fasting [source] |
Circadian rhythm and reference ranges
Reference ranges for cortisol and testosterone were established on morning samples, so an afternoon value can read 'low' simply because of the clock. The Endocrine Society's hypogonadism guideline specifically calls for morning, fasting samples and a repeat before diagnosing low testosterone. Cortisol is only interpretable if the draw time is known — always record it.
TSH varies less, but it does run higher overnight and lower in the afternoon; repeat tests at a consistent time to make small changes meaningful.
Common questions
What's the best time of day for a blood test?
Early morning, fasting, before medications you've been told to hold. It satisfies the strictest test on almost any order and lines up with how reference ranges were set.
Does afternoon testosterone count?
It can under-read, especially in men under 45. Guidelines call for a morning draw and, if low, a repeat morning draw before any diagnosis.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.