Trough Level: The Lowest Point Before Your Next Dose
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[Glossary · How doses act in the body]

Trough level

Also called trough, Ctrough, trough concentration, Cmin · Reviewed

Definition: Trough level is the drug concentration at the end of a dosing interval, just before the next dose is due. In the FDA's terms, Ctrough is the concentration at the end of the interval and Cmin is the lowest level within it; for most injections these land close together.

Why it matters when you track doses

The same person can get very different lab numbers depending on when blood is drawn. A draw the day after a shot sits near the peak; a draw right before the next shot sits near the trough. Without knowing which, a result is hard to read.

Different sources pick different timing. The Xyosted label measures the trough 7 days after the most recent dose and targets 350 to 650 ng/dL. The Endocrine Society guideline says to measure testosterone midway between injections for injectable enanthate or cypionate, and to adjust if that mid-interval level is above 600 or below 350 ng/dL. A midway draw is not a trough.

Worked example: timing a blood draw

Weekly shot every Sunday at 9 AM

  1. Trough timing (Xyosted style): 7 days after the last shot is the next Sunday, 9 AM, before that day's shot.
  2. Mid-interval timing (Endocrine Society style): half of 7 days is 3.5 days, so Wednesday around 9 PM. Labs are often open in the morning, so Wednesday or Thursday morning is the practical choice.
  3. For a shot every 14 days, midway is day 7.
  4. Write down the draw time and the last shot time together. Those two times are what your clinician needs to read the number.

Dose Track logs the date and time of every dose, so the gap between your last shot and a lab draw can be read straight off your history.

Common mix-ups

  • Trough vs mid-interval. They are different points; a target range only makes sense with its timing.
  • Before steady state, troughs keep rising. An early trough is lower than it will be later.
  • Late shot, lower trough. If you inject a day late, the trough you measure that week is a day longer into the decline.

Read more on Dose Track: Weekly vs twice-weekly TRT injections · Missed dose guides

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
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Frequently asked questions

When should a trough level be drawn?+
By definition, right before the next dose is due. Some testosterone guidance uses a mid-interval draw instead, so check which timing your target range assumes.
Why is my testosterone result lower than last time?+
One common reason is timing: a draw closer to your next shot catches a lower point in the cycle. Compare results only when the time since the last dose is similar.

Sources

  1. FDA guidance for industry: Bioavailability Studies Submitted in NDAs or INDs, General Considerations (April 2022)
  2. Xyosted (testosterone enanthate) US prescribing information, sections 2, 5.1, 6.1 and 12.3, DailyMed
  3. Bhasin S et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2018 (Tables 7 and 9). PMID 29562364

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