Does TRT actually cause hair loss, or is that genetics?

Genetics decide if you're going to lose your hair. Testosterone (through its metabolite DHT) decides how fast, and only in follicles that were already wired to respond to it.

How DHT actually causes hair loss

Testosterone gets converted to dihydrotestosterone (DHT) by an enzyme called 5-alpha reductase. DHT binds to androgen receptors in genetically sensitive hair follicles. That binding triggers mitochondrial dysfunction in the dermal papilla cells at the base of the follicle, pushing those cells into senescence - basically shutting down the cell's ability to keep producing hair. Do that enough times across enough growth cycles and the follicle shrinks (miniaturizes) until it's producing thin vellus hair instead of a full terminal strand. That's androgenetic alopecia (AGA). nih

The genetic side is bigger than most people think

Two independent twin studies put AGA heritability at around 80%. That's not a soft trend, that's one of the more heritable traits in dermatology. A newer analysis broke that down further: about 42% of the heritability in late-onset baldness and 56% in early-onset baldness traces to genes on the regular chromosomes, with another 10% and 23% respectively tied to the X chromosome. JidonlinePubMed Central

The androgen receptor gene itself sits on the X chromosome, which is part of why hair loss patterns run so strongly through the maternal side of a family. But it's not one gene doing the work. Research has also flagged WNT10A as a contributor, and it's understood now as a polygenic condition, meaning dozens of genes each contribute a small piece rather than one "baldness gene" calling the shot. Jidonline

One thing worth flagging if you're already thinking about TRT

Early-onset AGA has also shown up linked with obesity, insulin resistance, and cardiovascular disease risk in the research, which is a separate conversation from "will TRT thin my hair," but worth knowing if hair loss is already showing up early. That's a reason to get a broader metabolic panel done anyway, not just a hormone panel. PubMed Central

What this actually means for you

If your follicles don't carry the genetic sensitivity, more testosterone from TRT isn't going to touch your hairline. If they do, TRT can speed up a process that was already scheduled to happen. There's no version of this where TRT is "safe for hair" or "causes hair loss" as a blanket statement. It's conditional on what you inherited.

If hair is a concern going into TRT, get a baseline (photos, family history, and if you want to get specific, a genetic panel) before starting, then monitor it during treatment instead of trying to figure out the cause after the fact.

TLDR: TRT doesn't cause hair loss on its own. It can accelerate hair loss in follicles that were already genetically primed for it through DHT-driven follicle miniaturization. Heritability sits around 80%, so if your dad or maternal grandfather went bald early, that's the bigger predictor, not your future testosterone levels.

Anyone here start TRT worried about hair and actually track what happened before and after?

References

1. Androgenetic Alopecia in Men: An Update on Genetics. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC11305502/
2. Heilmann-Heimbach, S. et al. Androgenetic Alopecia: Identification of Four Genetic Risk Loci and Evidence for the Contribution of WNT Signaling to Its Etiology. Journal of Investigative Dermatology. https://www.jidonline.org/article/S0022-202X(15)36283-7/fulltext (cites the twin studies establishing ~80% heritability: Nyholt et al., 2003; Rexbye et al., 2005)
3. Cyanidin 3-O-arabinoside suppresses DHT-induced dermal papilla cell senescence by modulating p38-dependent ER-mitochondria contacts. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8900350/
4. Establishment and characterization of matched immortalized human frontal and occipital scalp dermal papilla cell lines from androgenetic alopecia. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10696020/