On Testosterone Therapy? These Are the Numbers That Actually Matter
Testosterone therapy is now common, easy to start, and frequently under-monitored. People check the number the therapy is named after and assume that is the job done. It is not.
The marker that most often changes the plan
Haematocrit — the proportion of your blood made up of red cells. Testosterone stimulates red-cell production, and in a meaningful minority of men on therapy the blood gradually thickens. A rising haematocrit is the single most common reason a prescriber lowers a dose, changes the delivery, or pauses treatment.
It is also the marker least likely to be on a cheap hormone panel, because it needs a different sample tube from the hormone assays. That is an operational reason, not a clinical one — and it is why we treat the tube question as a clinical decision rather than a logistics one.
What a proper monitoring panel covers
- Total testosterone — the headline number, and the least informative on its own.
- Free testosterone — the fraction actually available to your body. Calculated, not measured; see below.
- SHBG — the protein binding most of your testosterone. Without it, total testosterone is hard to interpret.
- Estradiol — testosterone converts to oestrogen, and the ratio matters more than either alone.
- LH and FSH — show whether your own production has shut down, which is what usually happens on therapy.
- PSA — standard prostate safety monitoring for men from around 40.
- Haematocrit and haemoglobin — the safety marker above.
- Lipids and HbA1c — therapy moves both, in both directions depending on the person.
Timing changes the answer
Testosterone is highest in the morning and varies through a dosing cycle. A sample taken on day two of an injection cycle and one taken on day ten are different tests of the same person. If you want results you can compare over time, collect at the same time of day and at the same point in your cycle, every time.
This is the most common reason two “identical” tests disagree, and it has nothing to do with the laboratory.
Free testosterone is calculated, not measured
On almost every panel you will ever see, free testosterone is not measured directly. It is derived from total testosterone, SHBG and albumin using a published equation. We use the Vermeulen calculation and we show you the formula and the three inputs that produced your number.
That matters because if your albumin or SHBG is unusual, your free testosterone is being calculated on unusual inputs — and you deserve to be able to see that rather than take a single figure on faith.
What this is not
LabX does not prescribe testosterone or advise on dosing. We run the laboratory work for people already under a prescriber’s care, and the results go to you and to whoever ordered them. Bring the numbers to your clinician; that conversation is the point of having them.