Testosterone replacement therapy (TRT) and resting heart rate/HRV: what changes and why

Three mechanisms, three different directions

Unlike a single-mechanism story (say, caffeine's stimulant effect), TRT's net effect on your wearable numbers is the sum of at least three things pulling differently, which is why the research doesn't agree on a single direction.

The first three rows are genuinely different studies measuring genuinely different things — a 9-week HRV trial, a 24-hour Holter safety study, and a multi-year observational registry aren't comparable head-to-head, and none of them is a clean, single-cause answer the way GLP-1 medications' effect on wearable data turned out to be. That inconsistency is the honest finding here, not a gap in the search.

Why "does TRT raise HRV" depends entirely on your starting point

The clearest signal in the literature is that testosterone therapy behaves like a deficiency-correction intervention, not a universal HRV booster. In the one controlled study measuring this directly — men with metabolic syndrome and a confirmed low testosterone level — 9 weeks of testosterone therapy raised HRV parameters that had been significantly suppressed relative to men without the deficiency, though the treated group's HRV still hadn't caught up fully to the non-deficient controls by the end of the study. That's the same shape as correcting iron-deficiency anemia: a real physiological problem improves when you fix the actual deficiency causing it, and the wearable number is downstream of that fix, not the target of the treatment itself.

That pattern doesn't automatically extend to every population taking exogenous testosterone. A 2025 study in transgender men receiving masculinizing hormone therapy — a genuinely different starting hormone range, treatment goal, and measurement protocol — found reduced autonomic modulation specifically during a psychological stress task, the opposite direction from the deficiency-correction result above. The honest read isn't that one study is wrong; it's that "testosterone and HRV" isn't one relationship, it's several, and which one applies to you depends on whether you're correcting a documented deficiency or starting from a different baseline entirely. This is worth remembering any time an HRV or resting-heart-rate trend shifts for a reason that isn't obviously training load, illness, or sleep — see the six real reasons an HRV baseline changes for the fuller list TRT belongs on.

The actual cardiovascular signal: erythrocytosis, not heart rate

If there's one thing worth tracking closely on TRT, it isn't a wearable metric at all — it's hematocrit, the percentage of blood volume made up of red blood cells. Testosterone stimulates red blood cell production, and in a meaningful minority of users this goes further than intended, producing secondary erythrocytosis: blood that's measurably thicker than normal. The largest and most rigorous safety trial to date, TRAVERSE (5,246 men with confirmed hypogonadism and elevated cardiovascular risk, testing testosterone against placebo), found erythrocytosis in 17.0% of the testosterone group versus 3.3% on placebo — by far the most reproducible adverse finding in the trial, well ahead of any signal on heart attacks or strokes, which showed no significant difference between groups overall.

Erythrocytosis doesn't reliably announce itself as an isolated resting-heart-rate spike on a wearable — the mechanism is thickened blood raising clotting and, at high levels, stroke-adjacent risk, not a direct pacemaker effect. That's exactly why it's tracked with a blood test rather than a graph on your phone. Standard monitoring calls for a baseline hematocrit before starting, rechecks at 3, 6, and 12 months, and annually after that if stable, with treatment typically adjusted to keep hematocrit at or below roughly 54% (some guidelines use a 50% threshold). Time-to-onset varies by cohort — some data shows the biggest rise in the first 3-12 months, other longer-term cohorts report a mean time-to-peak hematocrit closer to 2-3 years — which is the practical argument for sticking with scheduled labs well past the first few months rather than assuming you're clear once your early checks look fine.

Injections vs. gels: the delivery method changes the risk, not the wearable reading

Nothing about testosterone delivery method shows up differently on a wearable — a resting heart rate sensor can't tell an injection from a gel. But the underlying erythrocytosis risk genuinely differs by formulation, which is useful context for understanding why your prescriber's monitoring schedule might look the way it does. Injectable formulations, particularly shorter-acting esters given every one to two weeks, are associated with meaningfully higher erythrocytosis rates and higher peak hematocrit than gels or long-acting pellets — plausibly related to the higher peak testosterone levels injections produce between doses. Gels carry the lowest documented risk and have a practical safety advantage beyond the number itself: because nothing is sitting in a tissue depot, a gel can be paused immediately if a lab value climbs, while an injected dose has to be metabolized on its own timeline. None of this is a formulation recommendation — that decision involves cost, convenience, and clinical judgment your prescriber is better positioned to make — but it explains why "my hematocrit is being watched" isn't a one-size-fits-all schedule across different TRT regimens.

A practical way to track this without over-interpreting daily noise

1. Get a genuine pre-treatment baseline if you can — two to four weeks of resting heart rate and HRV before starting gives you something real to compare against, the same principle used for tracking GLP-1's effect on recovery data.
2. Watch the multi-month trend, not single days. The heart rate and HRV shifts described above play out over weeks to years, not overnight — a few beats per minute of drift over a quarter is consistent with the published data; a single unusual morning reading isn't diagnostic of anything.
3. Treat hematocrit as the number that actually matters, separate from anything your wearable shows. Keep your bloodwork on the schedule your prescriber sets (commonly baseline, then 3, 6, 12 months, then annually) regardless of how your recovery score or resting heart rate looks that week — the two aren't substitutes for each other.
4. Keep lab results and wearable trends in one place rather than scattered PDFs and screenshots. Vita's health-report reader is built to photograph a hematocrit or full blood-count panel and track it over time alongside your recovery score trend, so the number that actually predicts risk here doesn't get lost between quarterly visits.
5. Note your formulation and injection timing if you're tracking closely. If you're on a shorter-acting injectable, day-to-day HRV or resting-heart-rate variability across your dosing cycle is plausible given the peak-trough hormone pattern involved — that's a pattern to describe to your prescriber, not to self-diagnose.

When to bring it to a doctor rather than wait for your next scheduled check

Most of what's described above is either a modest, expected shift or a risk that's only visible on bloodwork rather than a wearable. Raise it sooner than your next scheduled visit if you notice:

• Headache, dizziness, blurred vision, or a persistently ruddy/flushed complexion — the classic symptom cluster of significant erythrocytosis
• Chest pain, sudden shortness of breath, or one-sided leg swelling — the pattern associated with the (numerically higher but not statistically significant in TRAVERSE) clotting-related signals seen with testosterone therapy
• A new, irregular, or racing heartbeat you can feel, rather than a wearable number quietly drifting
• A resting heart rate that keeps climbing for weeks with no plateau, rather than settling into a new, stable range

A wearable can show you that your resting heart rate or HRV moved. It can't tell you whether that movement reflects a deficiency finally being corrected, an early formulation effect, or something that needs a hematocrit check sooner than scheduled — that judgment belongs with whoever is managing your treatment, informed by labs a wrist sensor was never built to replace.