TL;DR

SHBG is a liver-produced protein that binds sex hormones and influences how much is readily available to your tissues
Oral oestradiol raises SHBG more than transdermal or injectable routes, due to first-pass liver exposure
SHBG-bound hormone is generally not readily bioavailable — but the biology is more complex than “locked away”
Current evidence does not show SHBG level itself predicts feminisation outcomes
SHBG is not part of standard monitoring, but calculating bioavailable testosterone using SHBG + albumin can be useful when suppression is inadequate

Sex hormone-binding globulin (SHBG) is a protein produced by the liver that binds to sex hormones — primarily testosterone and oestradiol — and influences how much of each is biologically available to your tissues. Understanding it matters if you’re on HRT, because total hormone levels on a blood panel don’t tell the full story.

How protein binding works

Sex hormones in the blood exist in three forms: tightly bound to SHBG, loosely bound to albumin, or unbound (free). For testosterone, the broadly accepted approximation is that only around 1–4% is free, with the remainder protein-bound. For oestradiol, approximately 37% is SHBG-bound, around 60% is albumin-bound, and roughly 3% is free — though these figures are approximations and vary with SHBG level, albumin concentration, assay method, and individual physiology.

Hormone tightly bound to SHBG is generally considered less readily available for receptor interaction. Free hormone and the fraction loosely bound to albumin are considered biologically accessible. For oestradiol specifically, the picture is somewhat more complex: SHBG-bound oestradiol is not simply “locked away,” as experimental evidence suggests SHBG itself may participate in membrane-associated signalling pathways, although the physiological significance of these findings remains uncertain. The practical implication is that SHBG-bound oestradiol should be understood as generally not readily bioavailable rather than entirely inert.

How HRT changes SHBG

Oral oestradiol substantially increases SHBG production. This is primarily a hepatic first-pass effect — oral oestradiol exposes the liver to high concentrations before entering systemic circulation, stimulating hepatic binding protein production. Non-oral routes (transdermal, injectable) largely bypass this first-pass effect and produce smaller increases in SHBG.

Androgens tend to suppress SHBG production, contributing to lower SHBG concentrations in androgen-dominant states. As testosterone levels fall during feminising HRT, SHBG typically rises.

Does SHBG affect feminisation outcomes?

The available evidence does not show that SHBG level alone predicts feminisation outcomes. Studies in trans women — including de Blok et al., which examined breast development across oestradiol quartiles in a 12-month prospective multicentre study — have found that higher oestradiol levels do not appear to enhance breast development or feminising body composition changes once testosterone is adequately suppressed. UCSF guidelines similarly note there is no evidence that higher oestradiol levels, in the context of adequate androgen suppression, produce additional feminisation.

The practical conclusion is not that SHBG is irrelevant, but that it is unlikely to be the rate-limiting factor in feminisation for most people on therapeutic oestradiol levels. High SHBG by itself should not be interpreted as evidence that HRT is failing.

What SHBG is useful for

SHBG is not part of standard Endocrine Society monitoring, which specifies oestradiol and total testosterone as the routine measures, with targets of oestradiol 100–200 pg/mL and testosterone below 50 ng/dL.

However, calculating bioavailable testosterone using SHBG and albumin can be informative in specific situations — particularly where testosterone suppression is inadequate despite oestradiol levels appearing therapeutic, or where there is suspected protein-binding abnormality. UCSF protocols include SHBG and albumin in some circumstances for calculating bioavailable testosterone for this reason.

⚠️ Clinical note: The binding percentages cited are population-level approximations. Individual values vary with SHBG level, albumin, assay method, and physiology. SHBG should be interpreted alongside the full panel, not in isolation.

Sources

Hembree WC, Cohen-Kettenis PT, Gooren L, et al. Endocrine Treatment of Gender-Dysphoric/Gender-Incongruent Persons: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2017;102(11):3869–3903. doi:10.1210/jc.2017-01658
de Blok CJM, Klaver M, Wiepjes CM, Nota NM, Heijboer AC, Fisher AD, Schreiner T, T’Sjoen G, den Heijer M. Breast Development in Transwomen After 1 Year of Cross-Sex Hormone Therapy: Results of a Prospective Multicenter Study. J Clin Endocrinol Metab. 2018;103(2):532–538. doi:10.1210/jc.2017-01927
UCSF Gender Affirming Health Program. Overview of Feminizing Hormone Therapy. University of California, San Francisco. Available at: transcare.ucsf.edu/guidelines/feminizing-hormone-therapy
Akinwumi BC, Martinie RJ, Bhambri A, Jayaraj A, Bhatt H, Bhatt M. Sex hormone-binding globulin provides a novel entry pathway for estradiol and influences subsequent signaling in lymphocytes via membrane receptor. Sci Rep. 2019;9(1):884. doi:10.1038/s41598-018-36882-3
Strauss JF, FitzGerald GA. Steroid Hormones and Other Lipid Molecules Involved in Human Reproduction. In: Strauss JF, Barbieri RL, eds. Yen & Jaffe’s Reproductive Endocrinology. 8th ed. Elsevier; 2019. [Estradiol binding fractions: ~38% SHBG, ~60% albumin, ~2–3% free; multiple independent sources converge on these approximations.]