SHBG and TRT: what the hormone math actually means

What did @trt__np actually say?

Nurse practitioner Vivian explained sex hormone-binding globulin (SHBG) as a liver-produced protein that binds testosterone in the bloodstream, reducing how much is biologically available. She argued that doctors who only check total testosterone miss the full picture, especially in older men. Her target SHBG range for TRT patients is 20 to 35 nmol/L. She also claimed that high-dose, infrequent injections crush SHBG and cause excess free testosterone, and that diabetes, thyroid deficiency, and steroid misuse lower SHBG into problematic territory. She endorsed calculated free testosterone over serum free testosterone assays, and recommended GLP-1 agonists like semaglutide for diabetic patients with low SHBG.

She also stated, somewhat confidently, that "a serum free testosterone is still what's floating around in your bloodstream" and that she doesn't care about it. That specific claim deserves scrutiny, which we'll get to.

Does the science back this up?

Mostly yes, with some real caveats. The foundational SHBG biology is accurate, but her dismissal of serum free testosterone testing and her specific target range are more opinion than established evidence.

SHBG binds testosterone with high affinity, leaving a fraction as free (unbound) or loosely albumin-bound, collectively called bioavailable testosterone. This is well-documented. A 2004 study by Vermeulen and Kaufman in the Journal of Clinical Endocrinology and Metabolism confirmed that elevated SHBG in older men can produce normal total testosterone readings while free testosterone is substantially reduced, producing symptomatic hypogonadism. That matches her clinical point exactly.

Her claim that diabetes lowers SHBG is supported by research. Insulin resistance suppresses hepatic SHBG production. A 2010 study by Ding et al. in Diabetes Care found that higher fasting insulin was independently associated with lower SHBG in men. The thyroid-SHBG connection is also real: hypothyroidism reduces SHBG synthesis, while hyperthyroidism raises it (Anderson, 2003, Thyroid).

Her claim that high-dose infrequent TRT injections suppress SHBG is directionally correct. Supraphysiologic testosterone transiently drives SHBG down. This is why many TRT clinicians prefer smaller, more frequent dosing.

What did they get wrong (or right)?

Her dismissal of serum free testosterone is the most contestable claim in the video, and it's worth pushing back on directly.

She said she doesn't care about serum free testosterone because it's "still floating around in your bloodstream." That framing is misleading. The issue with direct serum free testosterone assays is not that they measure the wrong thing, it's that most clinical labs use an analog immunoassay method that is notoriously inaccurate. The gold standard is equilibrium dialysis, which is expensive and not widely available. Calculated free testosterone using total testosterone, SHBG, and albumin is a reasonable validated proxy (Ly et al., 2010, Clinical Chemistry). But dismissing serum free T without explaining why the assay is flawed could confuse patients into thinking the concept of measuring free testosterone directly is worthless, when the problem is methodological, not conceptual.

Her SHBG target of 20 to 35 nmol/L is stated as a goal without citing evidence for those specific thresholds. That's not wrong, but it's clinical preference, not a guideline. The Endocrine Society's 2018 TRT guidelines do not specify an SHBG target range.

She gets credit for correctly identifying that older men, chronic illness, and polypharmacy raise SHBG, and that this can mask symptomatic hypogonadism on standard panels. That's a real and underappreciated clinical issue.

What should you actually know?

SHBG matters, but it's one variable in a hormone panel that requires clinical context. No single number tells the full story, including free testosterone.

• Total testosterone alone is insufficient for diagnosing hypogonadism. SHBG and albumin are needed to calculate bioavailable or free testosterone, per Endocrine Society guidelines (Bhasin et al., 2018, JCEM).
• Calculated free testosterone using the Vermeulen formula is considered more reliable than most direct immunoassay methods available in standard labs. This supports her practice, even if her explanation of it was incomplete.
• SHBG rises naturally with age, liver disease, hyperthyroidism, and certain medications including anticonvulsants and estrogens. It falls with insulin resistance, obesity, hypothyroidism, and exogenous androgen use.
• Injecting large testosterone doses infrequently does suppress SHBG and produces supraphysiologic peaks. Smaller, more frequent injections better mimic physiologic testosterone secretion and reduce these fluctuations (Nieschlag et al., 2012, European Journal of Endocrinology).
• GLP-1 agonists improving insulin sensitivity can raise SHBG in men with obesity-related low SHBG. This is a plausible mechanism, though robust TRT-specific outcome data for this combination is still limited.

Bottom line

This video is more clinically grounded than most TRT content on TikTok. The core SHBG biology is accurate. The clinical frustration with total-testosterone-only testing is legitimate. Where it gets shaky is in the dismissal of serum free T without explaining the assay problem, and in presenting a specific SHBG target range as settled science when it's really clinical convention. If you're on TRT or considering it, this video is a reasonable starting point, not a substitute for a provider who actually reviews your full panel.