Testosterone

Two things bend that scale before you read it. The first is the clock: because testosterone peaks in the early morning and slides through the day, a sample drawn at 8 a.m. and one at 4 p.m. on the same man are not the same test, which is why the morning draw is the standard. The second is the units. Most US labs report total testosterone in ng/dL, while much of the world uses nmol/L, and the two are not interchangeable (1 nmol/L is roughly 29 ng/dL). Read your value against the range printed beside it on the same report.

Low testosterone means the measured hormone is below the lab's lower limit, and in men this is the result the appointment was usually about. The difficulty is that the symptoms people attribute to it (fatigue, low libido, low mood, lost muscle, poor concentration) are common and have many other causes, so the number and the experience often only partly overlap.

The reasons a man's level runs low fall into a few groups:

• Age. Male testosterone drifts down slowly from roughly the thirties onward, on the order of about one percent a year on average, with wide individual variation.
• A problem at the testes themselves, from prior injury, certain infections, or genetic conditions.
• A problem upstream, in the pituitary or hypothalamus. Luteinizing hormone separates these two: high LH with low testosterone points at the testes, while low or normal LH alongside low testosterone points higher up. FSH usually rides on the same order and tells the parallel story for sperm production.
• A high prolactin, one of the pituitary causes doctors rule out, since it suppresses the signal that drives testosterone production and can quietly lower the level.
• Reversible drags: excess weight, poorly controlled diabetes, heavy alcohol use, chronic sleep loss, opioid medications, and the metabolic stress of illness. Sustained stress, reflected in markers like cortisol, can suppress the signal too.

This is also where the "low T" marketing gray zone lives. A gradual age-related decline is normal physiology, while clinics that promise to "optimize" every man back to youthful numbers are selling something the evidence doesn't support. The Endocrine Society frames treatment around men with both consistent symptoms and unequivocally low, confirmed morning levels, not around a number that dipped below a reference line once. The threshold for action is a doctor's judgment built on symptoms plus repeated testing, not a single figure and an advertisement.

High total testosterone reads very differently depending on who is tested. In men it is uncommon on a routine draw, and the usual explanation is an external source: testosterone therapy, anabolic steroid use, or certain supplements. It can also reflect dehydration concentrating the sample or, rarely, a hormone-producing tumor that warrants prompt investigation.

In women the question is sharper, because the female range is so much lower that a result that would be ordinary in a man is clearly elevated. The leading cause is polycystic ovary syndrome, in which the ovaries make more androgens than usual. NICHD describes elevated androgens as a core feature of PCOS, and the picture often includes irregular periods, acne, and unwanted hair growth. Less commonly it traces to the adrenal glands or a medication. When testosterone runs high in a woman, doctors often check DHEA-S to see whether the excess androgen is coming from the adrenal glands rather than the ovaries. When the question is adrenal rather than gonadal androgen output, clinicians often turn to androstenedione, the upstream intermediate that can run high even while total testosterone reads normal. A markedly raised testosterone in a woman is something to work up with a doctor, not to watch.

The reason total testosterone can mislead is that SHBG, the protein doing most of the binding, is not fixed. It rises with age, an overactive thyroid, and estrogen, and falls with excess weight, an underactive thyroid, and insulin resistance. When SHBG climbs, more of the collection moves into the locked section: the catalog total looks respectable while the usable, open-shelf share quietly shrinks. That mismatch is the most common reason a total testosterone and a man's symptoms fail to agree, and why a free testosterone or an SHBG measurement gets added when the story doesn't fit the number.

Testosterone is one line on a hormone panel, and it reads best beside the markers that explain it. LH and follicle-stimulating hormone tell you whether a low level comes from the testes or the signal above, while SHBG and free testosterone separate how much hormone exists from how much is available. In men, some testosterone is continuously converted into estradiol, so the two are often read together. Every androgen, testosterone included, traces back through the steroid pathway to pregnenolone, but topping up the precursor has not been shown to raise the hormones downstream of it. For male fertility, inhibin B reflects seminiferous-tubule activity more directly than testosterone, tracking with sperm count and testicular volume. The hormone panel guide shows how these lines fit into one picture.

Because testosterone swings with the time of day, sleep, and recent illness, a single value carries less weight than its direction over several confirmed morning draws. A total of 350 means one thing if it was 550 two years ago and something quieter if it has held at 350 the whole time. For a marker this easy to disturb on a single morning, trend direction often says more than any one result.