---
title: "Does the time of day change blood test results?"
type: prep
url: "https://www.test-well.com/prep/time-of-day"
factor: Time of day
aliases:
  - best time for blood test
  - morning vs afternoon blood draw
  - cortisol testosterone morning
reviewed_at: 2026-09-06
review_status: reviewed
date_published: 2026-09-05
---

# Does the time of day change blood test results?

> 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.


## Which results does it affect?

| Marker | Direction | How long the effect lasts | What to do | Source |
| --- | --- | --- | --- | --- |
| 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 | [Testing.com (AACC) — Cortisol](https://www.testing.com/tests/cortisol/) |
| 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 | [Endocrine Society — Testosterone therapy in men with hypogonadism (2018 guideline)](https://www.endocrine.org/clinical-practice-guidelines/testosterone-therapy) |
| TSH | raises or lowers | Highest late night/early morning; lower in the afternoon | Keep the draw time consistent between tests | [American Thyroid Association — Hypothyroidism / levothyroxine](https://www.thyroid.org/hypothyroidism/) |
| Iron (serum) | raises or lowers | Higher in the morning | Fasting morning draw | [NIH Office of Dietary Supplements — Iron fact sheet](https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/) |
| Prolactin | raises | Higher after waking and after meals, stress or exercise | Draw 2–3 hours after waking, rested, fasting | [Testing.com (AACC) — Cortisol](https://www.testing.com/tests/cortisol/) |

## 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.

## Related

- [Do you need to fast before a blood test — and which tests does eating affect?](https://www.test-well.com/prep/fasting)
- [Does biotin affect blood test results?](https://www.test-well.com/prep/biotin)
- [How does alcohol affect blood test results?](https://www.test-well.com/prep/alcohol)
- [Should you exercise before a blood test?](https://www.test-well.com/prep/exercise)
- [All preparation guides](https://www.test-well.com/prep)
- [Lab visit guide](https://www.test-well.com/at-the-lab)

## Sources

- [Testing.com (AACC) — Cortisol](https://www.testing.com/tests/cortisol/)
- [Endocrine Society — Testosterone therapy in men with hypogonadism (2018 guideline)](https://www.endocrine.org/clinical-practice-guidelines/testosterone-therapy)
- [American Thyroid Association — Hypothyroidism / levothyroxine](https://www.thyroid.org/hypothyroidism/)

---

Medically reviewed by Youmna DiStefano, MD (Medical Reviewer, TestWell); last reviewed 2026-09-06. Review policy: https://www.test-well.com/editorial-policy

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