SHBG and TRT: what the evidence says about binding proteins

What did @trt__np actually say?

Vivian, a nurse practitioner specializing in TRT, laid out a few rules for SHBG interpretation. Her main claims: SHBG should roughly equal your age in years before starting testosterone, low SHBG is primarily caused by insulin resistance, diabetes, and fatty liver, and "one big shot a week works for no one" because it "destroys your SHBG over time." She also argued that low SHBG on TRT causes testosterone to "blast your cells," leading to side effects that get misread as high estrogen.

She also suggested that long-term TRT users with SHBG in the teens is normal, but a rapid drop from 40 to 10 is the real danger zone. That distinction is worth examining separately because it's actually the most clinically useful thing she said.

Does the science back this up?

Partially, yes. The metabolic connection between low SHBG and insulin resistance is well-established. The age-equals-SHBG rule of thumb is not a published clinical benchmark, but it loosely tracks with observed population data.

SHBG does decline with testosterone use, particularly with supraphysiologic peaks from infrequent large injections. This is supported by pharmacokinetic research. Testosterone injections create high peak concentrations that suppress hepatic SHBG synthesis, and weekly large-dose injections produce more pronounced peaks than more frequent smaller doses. Platz et al. (2005, Cancer Epidemiology, Biomarkers and Prevention) and later work by Bhasin et al. (2010, Journal of Clinical Endocrinology and Metabolism) confirm SHBG's inverse relationship with insulin sensitivity and with exogenous androgen exposure. The claim that low SHBG increases the free testosterone fraction, thereby intensifying androgenic effects, is consistent with standard endocrinology. The framing that this gets misidentified as high estrogen is more speculative but not implausible.

What did they get wrong (or right)?

The "SHBG should equal your age" rule is presented as clinical fact. It is not. There is no published reference range that ties SHBG to age in this linear way. Standard lab reference ranges for adult men are typically 10 to 57 nmol/L regardless of age, though SHBG does trend upward with age in population studies. Calling this a rule of thumb is fine, but the video states it with a confidence that could mislead patients into thinking their lab is abnormal when it is not.

The claim that "high SHBG takes care of itself" is too casual. Persistently elevated SHBG can meaningfully reduce free testosterone bioavailability, and it does not always self-correct. Causes like liver disease, hyperthyroidism, or certain medications require investigation, not reassurance.

Where she gets real credit: the concern about large infrequent injections suppressing SHBG and skewing the free testosterone balance is clinically sound. The point about misattributing low-SHBG side effects to estrogen excess, and then incorrectly adding aromatase inhibitors, is a legitimate and underreported problem in TRT management. That part of the video is more useful than most TRT content on TikTok.

What should you actually know?

SHBG is a transport protein made by the liver. It binds testosterone and makes it biologically inactive while bound. Low SHBG means more free testosterone is available to act on tissues, which sounds good but creates a volatile hormonal environment when combined with large testosterone peaks from weekly injections.

The clinical consensus, reflected in Endocrine Society guidelines (Bhasin et al., 2018, Journal of Clinical Endocrinology and Metabolism), is that free testosterone or calculated bioavailable testosterone matters more than total testosterone when SHBG is abnormal in either direction. If your SHBG is very low, your total T number can look fine while your actual hormonal exposure is excessive.

The practical implication: dosing frequency matters, and a provider who only checks total testosterone once and never revisits SHBG is missing part of the picture. More frequent smaller injections, or other delivery methods, produce flatter pharmacokinetic curves and less SHBG suppression. That is not a fringe view. It is reflected in current clinical practice guidelines.

• Always get SHBG checked alongside total testosterone when evaluating TRT response.
• A SHBG in the teens is not automatically pathological on long-term TRT, but the trajectory and clinical symptoms matter.
• "High SHBG takes care of itself" is an oversimplification. Investigate the cause.