If you've been on testosterone replacement therapy (TRT) for more than a few months and your latest blood result has flagged a high haematocrit, you're in the most common — and most manageable — side effect of the entire treatment.
This guide explains what's happening, what the numbers actually mean, and how it's handled in New Zealand. It's written for men already on TRT and for men deciding whether to start.
What is haematocrit, and why does TRT raise it?
Haematocrit (HCT) is the percentage of your blood made up of red blood cells. The reference range for adult men in NZ labs typically sits around 0.40–0.50 (40–50%), with mild variation between providers.
Testosterone is a known erythropoietic stimulant — it tells your bone marrow to make more red blood cells. The effect happens through three overlapping mechanisms: a small rise in erythropoietin (EPO), suppression of the iron-regulating hormone hepcidin, and direct stimulation of marrow precursors. The rise typically plateaus 6–12 months into therapy.
For most men, the rise is mild and lands within the normal range or just above it. For some, it pushes high enough to need active management. This is secondary erythrocytosis (i.e. a treatable cause is identified — TRT — distinguishing it from primary polycythaemia vera, a different condition entirely).
What the thresholds actually are
International endocrine guidance and the BPACNZ November 2024 testosterone update converge on broadly the same thresholds. The most commonly cited cut-offs come from the European Academy of Andrology (EAA) 2020 position paper:
- HCT < 0.50: Normal range; no action needed.
- HCT 0.50–0.54: Watch closely. Recheck in 8–12 weeks. Look for reversible drivers (dose, hydration, sleep apnoea, smoking).
- HCT > 0.54: Action threshold. The EAA position paper, the Endocrine Society 2018 guideline, and consensus practice in NZ all flag 0.54 as the level at which intervention should be considered. Above 0.54, the risk of thrombotic events (DVT, PE, stroke, MI) is materially elevated.
- HCT > 0.60: Urgent. Pause TRT, exclude other causes, consider therapeutic venesection.
These numbers are guidance, not bright lines. A lean, fit, hydrated man with HCT 0.53 and no other risks is in a different position from a sedentary smoker with HCT 0.51, untreated sleep apnoea, and a family history of stroke. Numbers exist to provoke a conversation, not to replace one.
What raises haematocrit beyond the TRT itself
Before changing anything, it's worth ruling out the other things that drive HCT up:
- Untreated obstructive sleep apnoea — probably the biggest unrecognised contributor in TRT patients. Repeated nocturnal hypoxia drives EPO. If you snore, are tired during the day, or your partner has flagged you stopping breathing, get a sleep study.
- Smoking — chronic mild hypoxia plus carbon monoxide binding raises HCT independently of TRT.
- Dehydration on the day of the blood draw — apparent HCT rises if your plasma volume is low. Drink water normally on the morning of the test.
- Short-acting injectable formulations with high peak-to-trough swings (e.g. weekly or higher-dose enantate / cypionate) typically produce higher HCT than long-acting Reandron. Switching from short-acting to long-acting often drops HCT by 0.02–0.04.
- Higher-than-needed doses. A common pattern: TRT pushed into the high-normal or above-range testosterone band drives HCT harder than physiological replacement does.
- Altitude, polycythaemia vera, EPO use, COPD — rarer but should not be missed, particularly if HCT rises sharply or doesn't respond to TRT dose changes.
The first-line response in NZ practice
If you're on TRT and your HCT has climbed past 0.50, the standard sequence is:
- Repeat the blood test — fasted, hydrated, mid-cycle (not on a peak day). One isolated value isn't enough to act on if the trend isn't established.
- Address reversible drivers — sleep apnoea screening, hydration, smoking cessation conversation, review of any new medications.
- Adjust the TRT itself. Dose-down by 10–20% first. If you're on weekly enantate / cypionate at higher doses, splitting the dose into two smaller half-weekly injections often reduces peaks and lowers HCT. If a switch from short-acting to Reandron is feasible and clinically appropriate, that's a useful next step.
- Recheck in 8–12 weeks. Most men respond to dose adjustment without ever needing venesection.
When venesection is needed
If your HCT remains above 0.54 after dose changes, or if it's been above 0.60 at any point, therapeutic venesection (also called therapeutic phlebotomy) is the next step. The mechanism is simple: remove a unit of blood, the body makes new blood with the iron it has, and HCT drops.
In New Zealand there are two paths:
NZ Blood Service — High Ferritin Venesection Programme
NZ Blood Service runs a venesection programme that can be used for clinically indicated red-cell removal in patients who meet their criteria. The most common pathway in NZ for TRT-related secondary erythrocytosis is donor venesection through their programme — which has the advantage that the unit may be used for transfusion (i.e. you donate a useful product), and there is no charge to the patient.
To use this path:
- Your prescribing doctor refers you to NZ Blood Service with confirmation of the clinical indication.
- You need to meet standard donor eligibility criteria (age, weight, health, no high-risk behaviours by the donor questionnaire).
- A typical schedule is one unit every 8–12 weeks initially, then less often as HCT stabilises.
- Each unit removed lowers HCT by approximately 0.02–0.04 in a man of average size.
The eligibility window is the catch: men with conditions or medication histories that exclude blood donation (and TRT itself doesn't, but other histories may) cannot use the donor pathway.
Private therapeutic phlebotomy
If donor pathway eligibility doesn't apply, therapeutic phlebotomy is available privately through some NZ haematology clinics and a small number of private GPs. The procedure itself is unchanged: a single unit, taking around 20 minutes, with the same physiological effect. Pricing is set by the provider clinic and varies — ask the referring doctor for current options.
In both cases, iron studies are checked periodically. Repeated venesection drops ferritin; in TRT-related polycythaemia this is usually a feature rather than a bug — lower ferritin reduces the substrate available for ongoing red cell production — but iron deficiency anaemia is the failure mode at the other end.
Practical playbook for monitoring
A reasonable monitoring rhythm for a man on TRT, both for general safety and for catching HCT drift early:
- Baseline: full blood count, iron studies, ferritin, lipids, comprehensive hormone panel, PSA (where indicated).
- 6–12 weeks after starting or after any dose change: FBC, total testosterone (trough), estradiol.
- Every 3–6 months thereafter once stable: FBC (focus on HCT and Hb), trough testosterone, estradiol. Annual lipids, HbA1c, PSA where indicated.
- Annually: comprehensive review including iron studies (especially if you've had any venesection in the year).
If your HCT trends from 0.45 → 0.49 → 0.51 over consecutive checks, that's the signal to act early — much easier to head off at 0.51 than to manage at 0.56.
In plain English — what does this mean for you?
If you've just been told your haematocrit is "a bit high," here's what to know:
- It's common. Up to a third of men on TRT will see HCT rise into the upper end of normal or just above, particularly in year one.
- It is not the same as polycythaemia vera. That's a different, rarer disease. TRT-related rises are secondary and reverse with dose changes or venesection.
- It does not mean you need to stop TRT. In most cases your dose is adjusted, you give blood once or twice a year, and the issue goes away.
- Drink water normally on the morning of every blood test. A surprising number of false-high results are dehydration, not haematocrit.
- If you snore or feel unrefreshed by sleep, get checked for sleep apnoea. Treating sleep apnoea on its own can drop HCT meaningfully and improves nearly every other TRT-related marker.
- Don't go on a "TRT-friendly" supplement protocol you read about online. No supplement reliably lowers haematocrit. Hydration, dose adjustment and venesection do.
The tracking sheet you'll want to keep
Photograph or jot down each FBC result so you have your own trend line. The key columns:
- Date
- Haematocrit (HCT)
- Haemoglobin (Hb)
- Ferritin (when measured)
- Trough total testosterone
- Estradiol
- Current TRT dose and formulation
- Notes (recent illness, dehydration, recent travel, any venesection)
Trends matter more than any single number. Bring this with you to every follow-up.
Ready to get on top of it?
If you're already on TRT and your HCT is creeping up — or if your current prescriber isn't engaging with the issue properly — you can book a TRT review with Enhanced Men. We'll review your current protocol, your last 12 months of bloods, and put a monitoring and dose plan in place. Doctor-led, NZ-wide, evening and weekend appointments.
If you haven't started TRT yet, the TRT in New Zealand guide walks through the diagnosis pathway, costs, and what proper monitoring looks like from day one.
FAQ
What's a normal haematocrit on TRT? Within the lab reference range (around 0.40–0.50 for adult men in NZ). Most men on a well-titrated TRT protocol sit in the upper third of normal. Above 0.50 prompts close monitoring; above 0.54 prompts action.
Why is my haematocrit higher on TRT than before? Testosterone stimulates red blood cell production through EPO release, hepcidin suppression and direct marrow effects. The rise is dose-dependent and usually plateaus 6–12 months in.
Is high haematocrit dangerous? Above 0.54 the risk of thrombotic events (clots, stroke, heart attack) rises. Below that threshold, in an otherwise healthy man, the absolute risk is low. The point of monitoring is to catch and reverse the trend before it gets there.
Can I lower my haematocrit naturally? Hydration helps. Stopping smoking and treating sleep apnoea help meaningfully. No supplement reliably lowers HCT. Donating blood (or therapeutic phlebotomy) is the definitive treatment.
Will I have to give blood forever? No. Most men on stable, well-titrated TRT need at most one or two venesections a year, and many need none after the initial dose-titration period. Long-acting formulations (Reandron) often eliminate the need entirely.
Does Reandron cause less polycythaemia than enantate? Yes — generally. The lower peak-to-trough swing of Reandron (long-acting undecanoate) is associated with lower haematocrit rises than weekly or fortnightly enantate / cypionate at equivalent total doses.
Can I donate blood through NZ Blood Service to manage TRT-related HCT? Often yes, if you meet standard donor eligibility. Your prescriber can refer you. The donor pathway has no out-of-pocket cost and produces a usable transfusion unit. If you don't meet donor criteria, private therapeutic phlebotomy is the alternative.
What's the difference between TRT polycythaemia and polycythaemia vera? TRT-related polycythaemia is secondary — driven by an external stimulus (testosterone). Polycythaemia vera is primary — a clonal bone-marrow disorder, often associated with the JAK2 V617F mutation. The two are distinguished on history, examination and laboratory testing (typically EPO level and JAK2 mutation analysis if there's any uncertainty). Your prescriber can rule out PV if the picture isn't clear.
Should I stop TRT if my HCT is above 0.54? Not necessarily. The standard sequence is dose adjustment first, look for reversible drivers (sleep apnoea, hydration), then venesection. Stopping TRT outright is reserved for HCT > 0.60 or for men where dose changes and venesection haven't worked.
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Sources / further reading (verify before publishing)
- BPACNZ — Testosterone deficiency in adult men (November 2024 update): https://bpac.org.nz/2024/testosterone.aspx
- European Academy of Andrology (EAA) 2020 position paper on testosterone deficiency, including haematocrit threshold guidance
- Endocrine Society 2018 clinical practice guideline on testosterone therapy in men with hypogonadism
- NZ Blood Service — donor eligibility and high-ferritin / therapeutic venesection criteria: https://www.nzblood.co.nz/
- Medical Council of New Zealand — relevant standards on prescribing and ongoing care