Last reviewed: May 12, 2026 Last updated: May 12, 2026

Written by: Jay Hastings , CEO of PlexusDx

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance. His work has included scaling healthcare startups, leading CLIA lab integrations, and helping expand consumer access to precision health tools.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Science Liaison with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

This article is part of the PlexusDx Education Hub — your resource for evidence-based guidance on hormones and fertility. Browse all Hormones & Fertility education

Yes — and the mechanism is direct. SHBG (sex hormone binding globulin) variants that elevate protein expression or increase binding affinity reduce the free testosterone fraction regardless of how much total testosterone is present. On any androgen-pathway protocol that works by increasing testosterone levels, high-SHBG genetics mean the free testosterone response is blunted relative to the total testosterone gain. The protocol isn't failing. The genetics are intercepting the increase before it becomes bioavailable. The PlexusDx Precision Peptide Genetic Test analyzes SHBG variants as part of 14 pathways, 49 peptides, and 150+ genetic insights — so this variable is known before protocols begin rather than discovered after months of inadequate response.

How SHBG Genetics Shape the Free Testosterone Equation

Testosterone circulates in three states: tightly bound to SHBG (biologically inactive), loosely bound to albumin (weakly available), and free (fully bioavailable). The ratio of free to total testosterone depends directly on SHBG concentration and binding affinity — both of which are genetically influenced.

Well-characterized variants at the SHBG locus — including rs6259 (Asp327Asn), which reduces SHBG binding affinity, and other expression-regulatory variants — alter baseline SHBG levels independently of lifestyle, age, and hormonal environment. These aren't marginal effects: functional SHBG variants can produce substantially different free testosterone fractions at identical total testosterone concentrations. Two men at the same total testosterone level can have meaningfully different bioavailable testosterone based on nothing more than their SHBG genetics.

This matters profoundly for androgen-pathway protocols because total testosterone is what gets elevated; free testosterone is what drives androgen effects. A serum total testosterone reading does not tell a provider how much of that testosterone is actually available to cells — SHBG genetics determine that ratio.

The High-SHBG Scenario: When Protocols Under-Deliver

For men with high-SHBG genetic profiles, androgen protocols face a structural challenge: as total testosterone rises, SHBG continues to bind a large fraction of the increase, dampening the proportional rise in free testosterone. The provider observes a good total testosterone number; the patient may still experience suboptimal androgen effects — not because of protocol failure, but because SHBG is sequestering most of the gain.

Without knowing a man's SHBG genetics, a provider watching a muted clinical response may instinctively increase the dose further — potentially overshooting on total testosterone while still not achieving proportionally adequate free testosterone. This creates a situation where other protocol variables (estrogen, hematocrit) move in unwanted directions while the actual target — free testosterone — remains below where it needs to be.

Knowing SHBG genetics before protocols start changes this picture. A provider who understands a patient is genetically predisposed to high SHBG can set free testosterone — not total — as the primary monitoring target from day one, interpret lab results through the SHBG lens, and avoid the dose-escalation trap that high-SHBG men are most vulnerable to.

The Low-SHBG Scenario: A Different Set of Considerations

Low-SHBG genetic profiles present a different protocol landscape. With less SHBG binding, a larger fraction of circulating testosterone remains free — which amplifies androgen signaling at lower total testosterone levels. This can mean more pronounced effects at doses that would be modest for a high-SHBG man.

But higher free testosterone availability also means higher substrate for downstream conversions: more free testosterone for aromatase to convert to estradiol (CYP19A1 genetics determine the rate), and more for 5-alpha reductase to convert to DHT (SRD5A2 genetics determine that rate). A low-SHBG man with high-activity CYP19A1 faces a compounded estrogen management challenge — free testosterone rising quickly on modest dose increases, and a significant fraction converting to estradiol. Monitoring priorities differ substantially from the high-SHBG scenario, even when the protocols look similar on paper.

SHBG Is One of Six: What Else Interacts

SHBG genetics set the bioavailability dimension of testosterone — but free testosterone that clears the SHBG gate still has to navigate five other genetically variable dimensions before it produces a consistent androgen effect. The full interaction picture is covered in the Complete Guide to Genetic Men's Hormone Testing and the Why TRT Works for Some Men and Not Others: The Genetic Answer synthesis post.

Most relevant to SHBG in practice:

CYP19A1 — aromatase converts free testosterone to estradiol. Low-SHBG men have more free testosterone substrate available for aromatization, making CYP19A1 genetics especially consequential for their estrogen picture. CYP19A1 and Estrogen Conversion in Men covers this conversion variable in full.

AR (CAG Repeats) — receptor sensitivity determines how the free testosterone that does bypass SHBG is read at the cellular level. High-SHBG genetics and long AR CAG repeats in combination create a doubly attenuated androgen environment. Androgen Receptor CAG Repeats: Sensitivity Explained covers receptor sensitivity in full.

SRD5A2 — 5-alpha reductase converts free testosterone to DHT. Free testosterone availability (SHBG-dependent) shapes how much substrate SRD5A2 can work with. SRD5A2 and 5-Alpha Reductase Genetics covers the DHT conversion step. The deep-dive on SHBG itself — including its role across all six panel dimensions — is in SHBG Genetics: Why Your Free Testosterone Varies.

What Your SHBG Results Reveal

SHBG variant analysis from the Precision Peptide Genetic Test reveals your genetic baseline for SHBG expression and binding capacity — where your free-to-total testosterone ratio is genetically predisposed to sit before lifestyle, body composition, hormonal environment, and protocols layer on top. Results don't measure your current SHBG level; that requires a blood panel. They don't predict a specific free testosterone number. And they don't determine what protocol is right for you — those are clinical decisions made with a qualified provider.

What they deliver is a defined genetic starting point: the binding variable your provider needs to account for when setting protocol targets, choosing monitoring priorities, and interpreting the numbers that come back over time. Genetics as a guide — and for SHBG specifically, a guide that changes how total testosterone readings get read from the first lab draw forward.

The Precision Peptide Genetic Test analyzes how your genes influence hormone-related biological pathways. It does not recommend, prescribe, or determine which peptides you should use. Consult a qualified healthcare provider before beginning any peptide protocol.

Ready to understand your SHBG genetics before your next protocol conversation? Take the Precision Peptide Genetic Test

Frequently Asked Questions About SHBG Genetics and TRT

Does SHBG genetics directly affect free testosterone on androgen protocols?

Yes. SHBG variants that produce elevated expression reduce the free testosterone fraction regardless of total testosterone present. On androgen-pathway protocols, high-SHBG genetics mean a larger elevation is required to achieve equivalent free testosterone. The Precision Peptide Genetic Test analyzes SHBG as one of 6 Reproductive Health insights within 14 pathways, 150+ genetic insights.

Can SHBG levels be changed, or are they fixed by genetics?

SHBG levels are influenced by both genetics and modifiable factors. Genetics establishes a baseline expression tendency. Insulin resistance, elevated estrogen, thyroid status, and nutrition all modulate SHBG around that genetic floor. The Precision Peptide Genetic Test reveals the genetic component, which providers interpret alongside measured SHBG levels and clinical context.

Why do some men on androgen protocols still have low free testosterone?

High-SHBG genetics keep binding capacity elevated regardless of total testosterone. As total testosterone rises on any androgen protocol, SHBG continues binding a large fraction — limiting the proportional free testosterone gain. This is why providers managing SHBG-high men track free testosterone specifically, not total. The Precision Peptide Genetic Test identifies SHBG variants before protocols begin.

This article is part of the PlexusDx Education Hub. Browse all Hormones & Fertility education

Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance. References are included at the end of the article when scientific, medical, or health-related claims are discussed.

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Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.