What did @kmartfit actually say?
In a short video, @kmartfit draws a basic distinction between total and free testosterone. Total testosterone, they say, is "the total amount of testosterone that your body has inside of its system." Free testosterone is what is "bioavailable for your body to use" for muscle growth, hair growth, and bone density. The video ends by asking viewers to share their own numbers.
That's the whole claim. It's brief, it's non-technical, and it avoids the messiness of how testosterone is actually bound and transported in the bloodstream. Whether that simplicity serves the audience or misleads them is worth examining.
Does the science back this up?
Partially, yes. The core distinction is real and clinically relevant. But the explanation leaves out enough that it could give viewers a skewed picture of what "bioavailable" actually means.
Total testosterone is the aggregate of all circulating testosterone: testosterone bound tightly to sex hormone-binding globulin (SHBG), testosterone loosely bound to albumin, and the small fraction circulating freely. Free testosterone represents only the unbound fraction, typically around 1-3% of total testosterone in healthy adult men (Vermeulen et al., 1999, Journal of Clinical Endocrinology and Metabolism).
Here's the part @kmartfit skipped: albumin-bound testosterone is also considered bioavailable because albumin binds weakly and releases testosterone readily at the tissue level. This means "bioavailable testosterone" in a clinical context typically includes both free testosterone and albumin-bound testosterone, not just the free fraction alone. The Endocrine Society and most clinical guidelines distinguish bioavailable testosterone from free testosterone, treating them as related but not identical (Bhasin et al., 2018, Journal of Clinical Endocrinology and Metabolism).
What did they get wrong (or right)?
They got the headline right and the detail wrong. Crediting free testosterone as the fraction that matters for biological effects is scientifically defensible. Studies consistently show that free testosterone correlates better with symptoms of hypogonadism than total testosterone in men with abnormal SHBG levels (Rosner et al., 2007, Journal of Clinical Endocrinology and Metabolism).
What's misleading is the implication that free testosterone is the only bioavailable fraction. By saying free testosterone is "what is bioavailable for your body to use," the video effectively erases albumin-bound testosterone from the conversation. For a TRT audience, this matters. A man with low SHBG might have a low total testosterone but perfectly adequate bioavailable testosterone once albumin-bound hormone is included. Treating only on the free testosterone number in that scenario could lead to unnecessary intervention.
The listed benefits, including muscle growth, bone density, and hair growth, are accurate effects of testosterone action. Those aren't controversial. But framing only free testosterone as the driver oversimplifies the physiology.
What should you actually know?
If you're getting your testosterone levels checked, three numbers matter: total testosterone, free testosterone, and SHBG. SHBG is the binding protein that determines how much of your total testosterone is locked away and unavailable. High SHBG means more testosterone is bound tightly and less is circulating freely, even if your total number looks fine on paper.
Bioavailable testosterone, which includes the free fraction plus albumin-bound testosterone, is increasingly used in clinical settings as a more complete picture. Some labs calculate it using the Vermeulen equation rather than measuring it directly, and direct measurement methods have known reliability issues (Ly et al., 2010, Clinical Chemistry).
The takeaway: don't make treatment decisions based on total testosterone alone, and don't assume free testosterone tells the whole story either. A complete panel including SHBG gives you a more accurate read of what your body is actually working with. Talk to a clinician who interprets these numbers together, not in isolation.
Should TikTok viewers trust this video?
As a starting point for understanding why doctors order more than one testosterone test, this video is fine. It introduces a real and important concept without making dangerous claims or recommending doses or products. That's not nothing on a platform full of TRT content that does exactly those things.
But viewers who walk away thinking free testosterone is the only biologically active fraction are missing a piece of the picture that could actually affect their care. The omission of albumin-bound testosterone and SHBG context is a real gap, not a minor technicality. If you're navigating a hypogonadism workup or discussing TRT with a provider, bring the full panel, not just the two numbers this video mentions.