Hormone Testing: Why Timing Matters More Than Which Test You Book

Several hormones change predictably through the day or the menstrual cycle. A correctly performed test taken on the wrong day can be impossible to interpret. Booking the right date is often more important than choosing between panels. 

Why does timing change the result? 

Hormones are not steady. Some follow a daily rhythm driven by sleep and waking. Others rise and fall across the menstrual cycle by design. A number only means something when it is read against the point in that pattern at which the sample was taken. 

This is why a laboratory report can be entirely accurate and still tell you very little - the measurement is right, but there is no way to interpret it without knowing when it was taken. 

Hormones affected by time of day 

Testosterone follows a daily rhythm and is generally highest in the morning. Guidance for investigating low testosterone is to sample in the morning, and to repeat before drawing conclusions from a single result. An afternoon sample can read lower simply because of the hour. 

Cortisol has a strong daily pattern too, which is why cortisol requests usually specify a collection time. 

Hormones affected by cycle day 

For anyone with a menstrual cycle, several tests need a specific day. Progesterone is typically measured in the second half of the cycle, around a week before the next period is expected, because that is when it peaks. FSH and LH are usually measured in the early follicular phase, in the first few days of the cycle. 

Cycle length varies between people, so "day 21" is a convention rather than a rule. If your cycle is not 28 days, the right day shifts, and it is worth saying so when booking. 

What if your cycle is irregular or absent? 

Then cycle-day timing may not apply in the usual way, and the useful tests may be different ones. This is a case where it is worth talking to a clinician before booking rather than guessing at a date. 

What to tell us when you book 

If you are booking a cycle-dependent test, tell us the first day of your last period and your usual cycle length. If you are booking testosterone, book a morning slot. Two minutes at booking can prevent a wasted appointment and a result nobody can use.