GLP-1s and Men's Hormones

I have written at length about women’s hormones on these drugs and nothing about men’s, which is a gap I should have closed sooner. Men are roughly a third of the people I see on this medication and they ask about this constantly — usually at the end of the appointment, with a hand on the door.

The loop

Adipose tissue is not inert. It contains aromatase, the enzyme that converts testosterone into estradiol. More fat means more conversion, which means lower testosterone and higher estradiol, and the higher estradiol suppresses the pituitary signal — luteinising hormone — that tells the testes to make more.

So the sequence runs: more fat, less testosterone, less of the hormone that supports lean mass and metabolic rate, easier fat storage, more fat.

This is called obesity-related functional hypogonadism, and the important word is functional. The testes usually work. The signalling has been suppressed by something reversible.

What weight loss does

A meta-analysis of weight loss interventions found significant increases in both total and free testosterone, with the size of the increase tracking the amount of weight lost — bariatric surgery producing the largest rises, diet-induced loss producing smaller but genuine ones.

The pharmacological weight loss achievable now sits between those two. A 15% reduction is well within the range where meaningful hormonal change has been observed, and STEP 1 reported a mean 14.9% at 68 weeks.

Two honest caveats. The GLP-1 trials were not designed with testosterone as an endpoint, so most of this is extrapolation from the weight loss literature. And functional hypogonadism from obesity responds; primary testicular failure does not, and the two need telling apart with a proper assessment rather than a single low result.

The conversation about replacement

This is where I would want a man to slow down.

Testosterone replacement treats the number. Weight loss treats the cause. Both raise testosterone; only one addresses why it fell.

Replacement also has a consequence men are frequently not told about clearly: exogenous testosterone suppresses your own production and suppresses sperm output, often profoundly. It is a well-recognised and common cause of treatable male infertility, and recovery after stopping can take many months and is not guaranteed.

So: if fertility matters now, or might in five years, say so before the first prescription. That single sentence changes the plan, and it gets missed constantly because nobody asks a 34-year-old whether he might want children later.

A reasonable sequence for obesity-related low testosterone:

1. Confirm it properly — morning sample, repeated, with LH, FSH and SHBG
2. Treat the obesity
3. Recheck at 6 months
4. Reassess whether replacement is still indicated

A meaningful proportion of men no longer meet the criteria at step 4.

Erectile function

This improves for most men, and largely for vascular rather than hormonal reasons.

Erections depend on endothelial function, which is why erectile dysfunction is an early marker of cardiovascular disease — often preceding a cardiac event by years. It improves with the same things that improve arteries: better glucose, better blood pressure, better lipids, less visceral fat. A randomised trial of lifestyle-induced weight loss in obese men with erectile dysfunction found significant improvement.

Two practical points. New erectile dysfunction deserves a cardiovascular assessment, not just a prescription — the reasoning behind that is in the heart data. And as weight falls, the blood pressure medication contributing to it may itself need reducing, which is its own conversation.

Fertility, briefly

Obesity is associated with reduced sperm concentration, motility and normal morphology. Weight loss is generally associated with improvement, though the evidence is less consistent than for testosterone and sperm parameters are noisy measurements.

If you are actively trying to conceive:

• Testosterone replacement is the thing to avoid, for the reasons above
• Sperm production runs on roughly a 3 month cycle, so changes take at least that long to show
• Heat, alcohol and smoking matter, and alcohol may be easier to reduce than expected on this drug — drinking alcohol on a GLP-1
• There is no established requirement for men to stop a GLP-1 before conception, unlike the clear position for women, and that is a question for your own clinician rather than an assumption

The muscle question, which men ask differently

Men usually arrive at lean mass through appearance and leave through function, and the arithmetic is the same either way: a substantial share of weight lost on these drugs is lean tissue, and testosterone falling further would compound it.

Which is an argument for doing the protein and the training properly rather than for a testosterone prescription — 1.2–1.6 g/kg a day, 2 resistance sessions a week, from week one rather than from month six. That is muscle is the whole game, and the practical version is in the training rankings.

What I would ask for

• Morning testosterone, repeated, with LH, FSH and SHBG — a single afternoon total testosterone is close to useless
• The fertility question asked out loud before any replacement is discussed
• A recheck at 6 months rather than a decision at baseline
• Blood pressure medication reviewed as weight falls, since some of it contributes to sexual side effects
• New erectile dysfunction treated as a cardiovascular finding, not only a sexual one

And if you are already on testosterone replacement and losing weight, that dose is also worth reviewing. It was set for a different body.