My first short-form piece. One study. One resolution. One implication.
In December 2025, a team at Wakayama Medical University did what decades of pain research hadn't. They proved exactly where testosterone controls pain — and exactly where it doesn't.
They bred mice without androgen receptors in their sensory neurons. A conditional knockout targeting Nav1.8-expressing cells in the dorsal root ganglia. The result was modality-specific: mechanical pain thresholds dropped. Thermal pain — unchanged. Chemical pain — unchanged. Electrophysiology confirmed increased spinal dorsal horn neuron firing. The critical population: CGRP-positive peptidergic neurons, where androgen receptor depletion was greatest.
When they gave female mice dihydrotestosterone, mechanical thresholds rose to male levels. When they repeated this in AR-conditional-knockout females, nothing happened.
The gate was the androgen receptor. Sitting in sensory neurons. Tuning mechanical sensitivity and nothing else.
— Saika et al., PAIN, December 2025
Three Lines of Human Evidence Said No
This should have been headline news in endocrinology. It wasn't — because for decades, human data has been telling a different story.
In April 2025, a systematic review in PAIN Reports pooled 31 human studies on androgens and experimental pain. Meta-analysis: no relationship between testosterone levels and heat pain ratings. No relationship with electrical thresholds. Conclusion: "the current data do not support the effect of androgens on experimental pain sensitivity."
The only well-powered RCT — Basaria et al., 84 men with opioid-induced hypogonadism, 14 weeks of testosterone gel — found an odd split. Objective measures of pressure and mechanical hyperalgesia improved. Self-reported pain didn't budge.
And Tabernacki et al. (2025), analyzing 56,470 transgender men in a US clinical database, found those on testosterone therapy were 20% more likely to receive chronic pain diagnoses. Transgender men had nearly 6-fold higher fibromyalgia prevalence than cisgender women — despite testosterone.
Three lines of evidence. All negative or paradoxical. Until you read Saika.
The Mismatch
| Pain Modality | Saika (AR-cKO mice) | Human SR (31 studies) | Basaria RCT |
|---|---|---|---|
| Mechanical / pressure | Threshold ↓ | Underrepresented | Improved |
| Thermal (heat / cold) | No change | No effect | — |
| Electrical | — | No effect | — |
| Chemical (formalin) | No change | — | — |
| Self-reported clinical pain | N/A | N/A | No change |
The systematic review pooled studies measuring heat, cold, electrical, and pressure pain — with heat and electrical dominating. Saika proved testosterone gates mechanical pain only. Average across modalities and the signal dissolves into noise.
Basaria found exactly what Saika's mechanism predicts: pressure and mechanical hyperalgesia improved objectively. But self-reported pain captures everything — the chronic aching, the mood burden, the central sensitization. Testosterone gates a peripheral mechanical signal. It does not gate suffering.
The transgender paradox resolves the same way. Chronic pain in clinical databases includes fibromyalgia, migraine, visceral pain — conditions where AR-mediated mechanical analgesia in CGRP+ peripheral neurons is beside the point. Testosterone doesn't protect against all pain. It protects against one specific kind, in one specific cell type, via one specific receptor.
The evidence was never contradictory. It was never measuring the same thing.
Where the Gate Matters Most
The clearest implication is hiding in plain sight: opioid-induced hypogonadism.
63% of men on chronic opioids develop secondary hypogonadism. Testosterone drops. And Saika connects the next link: the androgen receptor in sensory neurons transcriptionally regulates mu-opioid receptor expression. Lose testosterone, lose MOR density. The opioids become less effective. Meanwhile, the mechanical pain gate opens — threshold drops.
The patient reports more pain. The dose goes up. The HPG axis suppresses further. A vicious cycle running in millions of patients — and fewer than 10% get screened for the hormonal deficit driving it.
Basaria proved this cycle is breakable: testosterone restored objective hyperalgesia measures in opioid-treated men. Saika explains why: restoring the androgen gate in CGRP+ sensory neurons, restoring mu-opioid receptor transcription. Buprenorphine partially sidesteps the problem by sparing the HPG axis.
But screening happens almost never. The gate stays open. The cycle runs.
Three studies. One mechanism. The androgen receptor in CGRP+ sensory neurons gates mechanical pain sensitivity. Decades of human research missed it because they tested the wrong modality. The patients paying the highest price — men on chronic opioids whose testosterone has been stolen by their own treatment — are the ones least likely to be screened for the deficit that would close the gate.
Sources
Saika F, Uta D, Fukazawa Y, et al. Androgen receptors expressed in the primary sensory neurons regulate mechanical pain sensitivity. PAIN. 2025;166(12):e746–e757. PubMed
Systematic review: The role of androgens on experimental pain sensitivity. PAIN Reports. 2025;10(2):e1262. Full text
Basaria S, Travison TG, Alford D, et al. Effects of testosterone replacement in men with opioid-induced androgen deficiency: a randomized controlled trial. Pain. 2015;156(2):280–288. PMC
Tabernacki T, et al. The burden of chronic pain in transgender and gender diverse populations. Eur J Pain. 2025;29(2):e4725. Full text
Berardi G, Sluka KA. Sex differences in the transition to chronic pain. J Clin Invest. 2025;135(11):e191931. PMC
Quinones M, Kadakia A, Bhatt S. Androgen receptor transcriptionally regulates μ-opioid receptor expression in rat trigeminal ganglia. Neuroscience. 2017;361:37–46. PMC
Kafel A, et al. Opioid-induced androgen deficiency in men. Andrology. 2025;13(3):e70013. Full text