Best Time to Draw Testosterone
Testosterone should be drawn in the morning (between 8-11 AM) after an overnight fast. 1
Rationale for Morning Fasting Sample
The Endocrine Society Clinical Practice Guideline explicitly states that serum testosterone concentrations exhibit significant diurnal variation with peak values occurring in the morning 1. This morning peak is critical for accurate diagnosis because:
- Glucose and food intake suppress testosterone concentrations 1
- Testosterone levels can vary substantially throughout the day due to circadian rhythms and episodic secretion 1
- A substantial fraction of older men (65-80 years) who have low afternoon testosterone will have normal morning levels 1
Important Caveats About Age
While aging reduces the magnitude of diurnal variation, the morning draw recommendation still applies to older men 1. Research has confirmed that even in men with a mean age of 61 years, testosterone levels drawn from 8-11 AM (411.7 ng/dL) differed significantly from those drawn outside this window (368.3 ng/dL, p=0.0003) 2. Afternoon samples (2-5 PM and 5-8 PM) showed the most significant reductions compared to morning draws 2.
Confirmation Testing
Always confirm an initial low testosterone result with a repeat morning fasting sample 1. This is critical because:
- 30% of men with an initial testosterone in the hypogonadal range will have normal levels on repeat measurement 1
- Day-to-day variations in testosterone are sufficiently large that a single measurement may be misleading 1
Special Considerations for Women
In premenopausal women, testosterone should be measured during the middle third of the menstrual cycle (follicular phase) in addition to morning timing 3, 4. DHEAS levels are less affected by timing and can be measured at any time 4.
Common Pitfalls to Avoid
- Do not draw testosterone in the afternoon or evening - levels from 2-8 PM are significantly lower and may lead to false-positive hypogonadism diagnoses 2
- Do not draw non-fasting samples - food intake suppresses testosterone 1
- Do not rely on a single low value - always confirm with repeat testing 1
- Do not ignore acute illness or medications - opioids, glucocorticoids, obesity, and systemic illness can all suppress testosterone independent of true hypogonadism 1