In 1849, a physician named Arnold Berthold transplanted testicular tissue between four roosters and concluded that the testes produce something that circulates in the blood and governs male characteristics.
In 2025, men apply a measured spray to their inner forearm or upper arm each morning and achieve physiological testosterone levels that are stable, predictable, and clinically meaningful — without visiting a clinic, without injection, and without the pharmacokinetic roller coaster that defined testosterone therapy for the first half-century of its clinical existence.
The distance between Berthold's roosters and the transdermal spray bottle is 175 years. It is also an education in how science moves: not in a straight line, not at a steady pace, and not without detours into monkey gland transplants, 15,000 liters of policeman urine, supraphysiologic injection peaks, and a Congressional hearing about gels accidentally absorbed by five-year-olds.
Let's go through the whole arc.
The Era Before the Molecule (Pre-1935)
For most of human history, the observable effects of the testes were known without any chemical explanation. Castration was practiced in ancient China, Rome, Egypt, and the Middle East — for political control of servants, vocal preservation of singers, and surgical management of prostate conditions. The eunuchs who ran the imperial bureaucracies of China demonstrated, across four thousand years of documented history, exactly what happens when testosterone is permanently removed from a male system. It was the world's longest involuntary clinical trial, and nobody called it that.
Berthold's 1849 experiment was the first to suggest a biochemical rather than neural mechanism. His conclusion — internal secretion — was correct. It took another 40 years for anyone to act on it clinically.
Brown-Séquard in 1889 injected himself with extracts of dog and guinea pig testes and reported remarkable rejuvenation to the French Academy of Sciences. He was probably experiencing a strong placebo effect. He was also, inadvertently, launching the organotherapy industry: a global market for glandular extracts and injectable organ preparations that would persist well into the 20th century. The science was mostly wrong. The demand — men wanting something to reverse the physical decline of aging — was very real, and it has never gone away.
Voronoff arrived in the 1920s with something more dramatic: actual tissue grafts. Chimpanzee and baboon testicular slices, sutured onto the testes of older men, ostensibly integrating and restoring testosterone production. He performed over a thousand such surgeries. He was celebrated by the Royal Society. Then the Royal Society had second thoughts, and he died in professional disgrace, his name briefly attached to a wave of fraudulent sheep-gland operations before fading from the record. The surgeries did not work. The demand they represented was legitimate.
1935: The Molecule Arrives
Everything changed when the molecule arrived with a name, a structure, and a synthesis route.
Ernst Laqueur's team in Amsterdam isolated testosterone from 100 kilograms of bull testes, yielding 10 milligrams of pure hormone. Adolf Butenandt and his colleague Hanisch in Göttingen synthesized it from cholesterol. Leopold Ruzicka and Wettstein in Basel did the same, independently, within weeks. Three breakthroughs. One molecule. One year.
The 1939 Nobel Prize in Chemistry went to Butenandt and Ruzicka for work on sex hormones. The Nobel Committee was right about the significance. Almost nobody noticed, because Germany had just invaded Poland.
What 1935 gave medicine was not just the molecule but the manufacturing pathway. Cholesterol — abundant, cheap — as the starting material. Chemistry as the means of production. The era of extracting hormones from biological sources in milligram quantities was over. Industrial production was possible.
The Injection Era (1937–1990s)
The first clinical delivery form was subcutaneous pellets: compressed crystalline testosterone implanted under the skin, releasing slowly over months. They worked, but insertion required a minor surgical procedure that limited uptake.
What dominated was injection. Testosterone propionate, requiring injections every two to three days, was the standard of care for years. Testosterone enanthate, introduced in the mid-1950s, stretched the interval to two to four weeks and became the dominant form of TRT for nearly half a century.
The problem — understood in detail by 1990 — was pharmacokinetics. After injection, serum testosterone spikes to supraphysiological levels within 36 to 48 hours, then falls through the physiological range toward the subtherapeutic range by days 21 to 28. Men spent portions of each injection cycle experiencing the effects of hormone excess and hormone deficiency in alternation. The mood swings, energy fluctuations, and libido variability associated with this pattern were real and well-documented.
The 1990 WHO/NIH/FDA workshop consensus was frank: the goal of testosterone therapy — maintaining physiological serum testosterone levels — was not being achieved by any available preparation. The standard of care had been the standard for decades. It was pharmacologically crude.
Meanwhile, the pharmaceutical industry had spent those same decades searching for better molecules rather than better delivery. By 1956, 256 distinct androgenic steroids had been synthesized. By 1976, more than a thousand. The pure anabolic compound — separating muscle-building from masculinizing effects — was never found. What the industry found instead was a massive toolkit for athletic doping and a long list of compounds with variable hepatotoxicity and unpredictable side effect profiles.
The Skin Solution (1990s–2010s)
The transdermal insight was not complicated in principle: the skin absorbs testosterone continuously, mimicking natural daily production, avoiding first-pass liver metabolism, providing stable serum levels rather than injected peaks and troughs.
The first transdermal testosterone was a scrotal patch, 1992, that required shaving and daily adhesion to scrotal skin. Effective, uncomfortable, and sufficiently off-putting that uptake was limited.
Non-scrotal patches followed in 1995, eliminating the body site problem. They introduced a new one: skin irritation in approximately 48% of users, with 12% experiencing severe local reactions.
AndroGel (1% testosterone gel) arrived in 2000 and changed the market overnight. A clear gel applied once daily to shoulders and upper arms, achieving stable testosterone levels throughout the day. By 2010, AndroGel was generating over $1 billion in annual sales. It also transferred readily to female partners and children through skin contact, producing virilization effects in children exposed inadvertently. The FDA issued a Boxed Warning.
The second-generation transdermal products — Axiron (axillary application), Fortesta (thigh), Natesto (intranasal) — each addressed specific limitations while introducing their own trade-offs.
The Spray Era (2010s–Present)
The transdermal spray represents the current state of the evolution. Metered-dose spray technology applied to testosterone delivery produces several advantages over gel formulations.
Volume is smaller: a measured spray delivers testosterone in microliters rather than milliliters of gel, reducing the surface area of skin with transferable hormone. Drying time is faster: approximately 60 seconds rather than 3 to 5 minutes for gel. The smaller application area and faster drying window dramatically reduce transfer risk compared to gel formulations.
Dosing precision is higher: a calibrated pump delivers a consistent measured dose with each actuation, without the volume variability that affects user-metered gel applications.
The compounding of transdermal testosterone spray at physician-specified strengths allows personalized dosing that mass-manufactured products cannot achieve.
The Full Accounting
Look at the whole timeline and count what it took: four roosters in 1849 to establish that the testes produce something hormonal. Forty years of empirical organotherapy, mostly ineffective but establishing patient demand. One hundred kilograms of bull testes to yield ten milligrams of testosterone. A Nobel Prize announced during an invasion. Fifty years of pharmacologically crude injections. A scrotal patch nobody wanted to use. A gel that worked brilliantly until it virilized the children. A 1990 workshop consensus that took thirty more years to fully act on.
And at the end of this: a small pump bottle, applied to the inner forearm or upper arm in the morning, delivering a measured dose of the same molecule Laqueur named in 1935, achieving the stable physiological testosterone levels that the WHO/NIH/FDA described in 1990 as the goal nobody had yet achieved.
The molecule has not changed. The biology has not changed. What changed was the delivery, the monitoring, the clinical framework, and eventually the access.
The rooster-to-spray-bottle arc is 175 years long. The destination — stable physiological testosterone replacement, accessible to men who need it, monitored appropriately, delivered reliably — is where we are now. It took longer than it should have. Medicine usually does.
This concludes The T Files — 50 posts, 10 series, 175 years of testosterone science. The history is long. The molecule is simple. The patients have been waiting the whole time.
