What did @kmartfit actually say?
@kmartfit laid out a basic framework: total testosterone measures everything circulating in your blood, while free testosterone is "what your body is actually able to use." The culprit tying up the rest? Sex hormone binding globulin, or SHBG. The practical punchline: "If you have a high total testosterone and a low free testosterone, you may still experience symptoms of low testosterone." That's the core argument, and it's worth stress-testing.
The explanation is short, TikTok-paced, and leaves out a lot. But the bones of it are accurate enough to be worth taking seriously rather than dismissing as bro-science.
Does the science back this up?
Yes, broadly. SHBG binds testosterone with high affinity, rendering most of it biologically inactive. What's left unbound, plus a smaller albumin-bound fraction that dissociates easily, constitutes what labs report as "free" or "bioavailable" testosterone. This isn't controversial physiology.
The clinical relevance is more debated. Vermeulen et al. (1999, Journal of Clinical Endocrinology and Metabolism) established the calculated free testosterone formula still widely used today, and noted that SHBG variability makes total testosterone an unreliable standalone marker in many patients. Later work by Bhasin et al. (2018, JCEM), including the Endocrine Society's clinical practice guidelines, acknowledged that men with elevated SHBG can have normal total testosterone but low free testosterone, and may present with hypogonadal symptoms. So @kmartfit's core point has guideline-level support, not just gym-floor consensus.
What did they get wrong or right?
They got the fundamentals right. SHBG does bind testosterone. Free testosterone is biologically active in a way that total testosterone is not. The symptom scenario they describe, high total with low free, is a real clinical pattern.
Where it gets slippery: the video implies SHBG only leaves a "portion" of testosterone usable, which is accurate, but it skips that albumin-bound testosterone is also partially bioavailable. Calling only the unbound fraction "what your body is actually able to use" is an oversimplification. Bioavailable testosterone, which includes albumin-bound hormone, is considered by many endocrinologists to be a more complete picture than free testosterone alone (Rosner et al., 2007, Journal of Sexual Medicine).
Also absent: the fact that free testosterone measurement itself is messy. Direct immunoassay tests for free testosterone are notoriously inaccurate. Equilibrium dialysis is the gold standard but rarely used in routine labs. Calculated free testosterone from total T, SHBG, and albumin is the practical workaround, but it introduces its own error margin.
What should you actually know?
If you're on TRT or evaluating whether you need it, total testosterone alone is genuinely insufficient for the full picture. Your SHBG level matters. High SHBG, which can be driven by aging, liver disease, hyperthyroidism, or certain medications, will eat into your free fraction even when total T looks fine on paper.
That said, don't assume a "low free T" number automatically explains your symptoms. Symptom attribution in men is complicated. Fatigue, low libido, and mood changes have long differential diagnoses. The Endocrine Society (Bhasin et al., 2018) recommends against diagnosing androgen deficiency on lab values alone, without consistent clinical symptoms. A single video, however accurate on the biochemistry, can't replace that clinical workup.
One more thing: if your lab uses a direct immunoassay for free testosterone, ask whether equilibrium dialysis or a calculated method was used instead. The difference can meaningfully change the interpretation of your results.
Bottom line: is this worth your attention?
For a 15-second TikTok explanation of a genuinely nuanced topic, this holds up better than most. @kmartfit isn't wrong. They're incomplete, which is a different problem. The SHBG-free testosterone relationship is real, clinically documented, and under-discussed in routine primary care. If this video pushes someone to ask their doctor for a more complete hormone panel rather than just a total testosterone number, that's a net positive outcome from a net oversimplified video.