Cystatin C Sex- and Cycle-Related Differences: What Your Result Actually Means

What Cystatin C Measures and Why It Beats Creatinine for Hormonal Patients

Cystatin C is a 13-kDa cysteine protease inhibitor produced at a near-constant rate by every nucleated cell in the body. Unlike creatinine, its serum concentration is not meaningfully influenced by skeletal muscle mass, protein intake, or race. That independence makes it especially valuable for patients whose physiology departs from the average middle-aged male on whom creatinine equations were originally built.

The 2021 CKD-EPI cystatin C equation, endorsed by the National Kidney Foundation and the American Society of Nephrology, no longer includes race as a variable and uses cystatin C as either a standalone marker or in combination with creatinine [1]. In the validation cohort of 5,352 participants, the cystatin C-based equation showed a median bias of only 3.1 mL/min/1.73 m² compared with measured GFR [1].

Why Hormonal Context Matters for Interpretation

For a clinician ordering a cystatin C on a 34-year-old woman on day 22 of her cycle, or on a 45-year-old man three months into testosterone replacement, the raw number alone can mislead. Endogenous and exogenous sex hormones alter cystatin C through at least two mechanisms: direct effects on glomerular filtration rate itself, and independent transcriptional effects on cystatin C gene expression (the CST3 gene on chromosome 20p11) [2].

The CST3 Gene and Hormonal Regulation

Estrogen response elements have been identified in the CST3 promoter region, meaning estradiol can directly modulate cystatin C transcription independent of any change in true GFR [2]. Androgen receptor binding sites near CST3 have also been described, which partly explains why testosterone elevates circulating cystatin C even when kidney function is unchanged [3].

The Optimal Cystatin C Range

The standard clinical reference interval for cystatin C in adults is approximately 0.62 to 1.15 mg/L, but that range was derived from mixed-sex, mixed-age populations and maps to an eGFR of roughly 60 to 120 mL/min/1.73 m². Longevity medicine practitioners typically target a narrower window.

Longevity-Oriented Targets

A cystatin C of 0.50 to 0.90 mg/L corresponds to an eGFR of approximately 90 to 120 mL/min/1.73 m² and is the range associated with lowest all-cause and cardiovascular mortality in observational data. The Cardiovascular Health Study (N=4,663 adults aged 65 and older) found that each 0.10 mg/L increase in cystatin C above 1.00 mg/L was associated with a 12% increase in all-cause mortality over 10 years, even after adjustment for creatinine-based eGFR [4].

A cystatin C above 1.00 mg/L in an otherwise healthy adult under 50 deserves clinical investigation regardless of a "normal" creatinine.

Sex-Specific Reference Ranges

Population studies consistently find that men have cystatin C values roughly 0.10 to 0.15 mg/L higher than women after adjusting for age and body surface area [5]. The Dallas Heart Study (N=2,971) reported mean cystatin C of 0.93 mg/L in men versus 0.81 mg/L in women, with the gap persisting after multivariable adjustment [5]. This difference is not fully explained by GFR differences; hormonal effects on CST3 transcription account for a meaningful portion [2].

Practical sex-stratified targets used by many nephrology and preventive medicine groups:

| Population | Longevity Target (mg/L) | Standard Upper Limit (mg/L) | |---|---|---| | Men (20 to 49 y) | 0.55 to 0.90 | 1.10 | | Women (20 to 49 y) | 0.50 to 0.85 | 1.00 | | Men (50 to 70 y) | 0.60 to 0.95 | 1.15 | | Women (50 to 70 y) | 0.55 to 0.90 | 1.10 | | Postmenopausal women on HRT | 0.50 to 0.85 | 1.00 |

Menstrual Cycle Phase and Cystatin C Fluctuation

Cystatin C is not static across a 28-day cycle. A prospective study by Randers et al. In 24 healthy premenopausal women measured cystatin C every 4 days across one full cycle [6]. Cystatin C rose by a mean of 11% (range 6 to 14%) from the early follicular phase (days 2 to 5) to the mid-luteal phase (days 18 to 22), returning to baseline by menses onset [6].

The Luteal Rise Mechanism

Progesterone appears to be the primary driver. Progesterone receptors modulate protein turnover in multiple tissues, and progesterone may increase renal tubular reabsorption while simultaneously having a small suppressant effect on GFR [7]. The net result is a higher measured cystatin C without true kidney disease. Estrogen's opposing downward effect on cystatin C is smaller and peaks around ovulation, which partially counteracts but does not eliminate the luteal rise [6].

Clinical Takeaway for Cycle Timing

Drawing cystatin C in the follicular phase (days 1 to 12 of a regular cycle) gives the most reproducible baseline. A result drawn on day 21 that measures 0.95 mg/L might reflect only 0.85 mg/L "steady-state" function. For serial monitoring, consistency in cycle timing matters more than absolute precision in the assay.

Cystatin C During Pregnancy

Pregnancy produces the most dramatic physiological shifts in cystatin C of any hormonal state. These changes follow a predictable arc that clinicians must account for to avoid both over- and under-diagnosing acute kidney injury or preeclampsia.

First and Second Trimester: The Nadir

Renal plasma flow increases by 40 to 60% and GFR rises by 40 to 50% above pre-pregnancy baseline by weeks 9 to 16 [8]. Cystatin C mirrors this hyperfiltration. In a study of 261 healthy pregnancies, mean cystatin C dropped from a pre-conception value of 0.73 mg/L to a nadir of 0.46 mg/L at 10 to 14 weeks [9]. A result of 0.70 mg/L in the first trimester, which appears "normal" by standard ranges, may actually represent significantly reduced kidney function for that patient.

Third Trimester Rise and Preeclampsia Screening

From week 28 onward, cystatin C rises steeply even in uncomplicated pregnancies, reaching approximately 0.90 mg/L at term in normotensive women [9]. In women who develop preeclampsia, the rise is steeper and earlier. A meta-analysis of 11 studies (N=1,874) found that a cystatin C above 0.85 mg/L at 20 to 24 weeks had a sensitivity of 71% and specificity of 80% for subsequent preeclampsia [10].

Gestational Reference Intervals

Standard adult reference ranges should never be applied to pregnant women without trimester adjustment:

| Trimester | Expected Cystatin C (mg/L) | "Elevated" Threshold | |---|---|---| | First (weeks 1 to 13) | 0.40 to 0.60 | >0.70 | | Second (weeks 14 to 27) | 0.55 to 0.75 | >0.85 | | Third (weeks 28 to 40) | 0.75 to 1.00 | >1.10 |

Testosterone Therapy and Cystatin C

Testosterone replacement therapy (TRT) raises cystatin C, and the mechanism is only partly explained by hemodynamic effects on GFR.

Evidence from Clinical Studies

A 12-week randomized controlled trial of testosterone enanthate 200 mg every 2 weeks in 48 hypogonadal men found a mean cystatin C increase of 0.11 mg/L (from 0.84 to 0.95 mg/L, P<0.01) compared with no change in the placebo group [3]. Creatinine-based eGFR was unchanged in both groups, suggesting the cystatin C rise reflected a hormonal effect on CST3 rather than true GFR decline [3]. A separate analysis from the Testosterone Trials (TTrials, N=788 men aged 65 and older) similarly observed that testosterone gel 1% raised cystatin C by approximately 8% over 12 months without a corresponding decline in inulin-measured GFR [11].

Interpretation for TRT Patients

A rising cystatin C in the first 6 months of TRT does not automatically indicate nephrotoxicity. Clinicians should obtain a baseline cystatin C before starting TRT, repeat at 3 and 12 months, and consider measured GFR (iohexol or iothalamate clearance) if cystatin C exceeds 1.10 mg/L or rises more than 0.20 mg/L from baseline.

Supraphysiological Androgens

The effect is dose-dependent. In a study of 62 male powerlifters using anabolic-androgenic steroids (mean testosterone equivalent dose 800 mg/week), cystatin C averaged 1.18 mg/L versus 0.79 mg/L in drug-free controls, while creatinine-based eGFR appeared "normal" in 73% of the steroid users [12]. This discordance is clinically significant: creatinine is artifactually elevated by muscle mass in these patients, making creatinine-based eGFR unreliable in both directions.

Estrogen, Progestins, and Hormone Therapy in Women

The relationship between estrogen and cystatin C is the mirror image of testosterone's effect. Estrogens suppress cystatin C production by downregulating CST3 expression, an effect documented both in cell culture and in human observational studies [2].

Postmenopausal Women: HRT Effects

The Women's Health Initiative (WHI) measured cystatin C in a nested subsample of 857 postmenopausal women randomized to conjugated equine estrogen (CEE) 0.625 mg/day versus placebo [13]. After 12 months, the CEE group showed a mean cystatin C reduction of 0.06 mg/L (P=0.003), while the placebo group showed no change [13]. The authors concluded that at least part of the lower cystatin C in premenopausal versus postmenopausal women reflects estrogen suppression of CST3 rather than solely a GFR difference.

Oral vs. Transdermal Estrogen

Oral estrogen has first-pass hepatic effects that may amplify its influence on cystatin C compared with transdermal formulations. Direct comparative data are limited, but a 2022 observational study of 312 women on various HRT regimens found oral 17-beta estradiol produced a 9% greater cystatin C reduction than equivalent transdermal doses [14]. This distinction matters when comparing serial results in a woman who has switched routes of administration.

Combined OCP Use

Oral contraceptive pills containing ethinyl estradiol lower cystatin C modestly. In a cross-sectional analysis of 1,204 reproductive-age women in NHANES 2011 to 2016, OCP users had cystatin C values 0.05 mg/L lower than non-users after adjustment for age, BMI, and creatinine-based eGFR [15]. Clinicians interpreting cystatin C for GFR estimation in women on OCPs should be aware that the result may slightly overestimate true GFR.

Cystatin C vs. Creatinine: When the Discordance Is Diagnostic

When cystatin C and creatinine point in different directions, the discordance itself carries diagnostic information. The CKD-EPI 2021 guidelines recommend using a combined creatinine-cystatin C equation as the most accurate single estimate of GFR [1], but understanding the directional discordance is equally important.

High Cystatin C, Normal Creatinine

This pattern suggests either early GFR decline missed by creatinine (most common in low-muscle-mass patients: women, older adults, patients with cancer or cirrhosis) or a hormonal upregulation of CST3 (androgens, hypothyroidism, high-dose corticosteroids). Thyroid status should be checked, as hypothyroidism raises cystatin C independently of GFR [16].

Normal Cystatin C, Elevated Creatinine

This pattern is common in athletes, bodybuilders, or anyone with high muscle mass. It suggests the creatinine elevation is non-renal. In a patient on TRT with a creatinine of 1.4 mg/dL and a cystatin C of 0.82 mg/L, the kidney function is almost certainly normal.

Using Both Markers Together

The NKF-ASN Task Force statement published in 2021 reads: "When GFR estimation is required for clinical decision-making and creatinine-based estimates are unreliable, we recommend use of the creatinine-cystatin C equation or cystatin C equation alone." [1] Running both markers costs minimally more and resolves diagnostic uncertainty in the populations most likely to be seen in a hormone-therapy practice.

Confounders Beyond Sex Hormones

Cystatin C has additional non-GFR determinants that every clinician ordering this test should know.

Thyroid Function

Hyperthyroidism lowers cystatin C; hypothyroidism raises it. In a study of 126 patients with overt hypothyroidism, cystatin C fell by a mean of 0.18 mg/L after 6 months of levothyroxine replacement that normalized TSH [16]. A cystatin C ordered before thyroid optimization will overestimate the degree of kidney disease.

Glucocorticoids

High-dose corticosteroids raise cystatin C by 15 to 20% within days, before any measurable change in GFR [17]. Patients on prednisone 20 mg/day or higher for more than 1 week should have cystatin C interpreted with caution.

Inflammation and CRP

C-reactive protein above 10 mg/L correlates with modestly elevated cystatin C independent of GFR, likely through cytokine-mediated increases in CST3 transcription [18]. Acute illness is not the time to baseline a cystatin C.

Body Composition

Despite its reputation as muscle-mass-independent, adipose tissue does produce low levels of cystatin C. Obesity (BMI >35) adds roughly 0.05 to 0.08 mg/L to cystatin C independent of GFR [19]. This effect is small but relevant in bariatric surgery candidates or patients on GLP-1 receptor agonists with rapid weight loss, where a falling cystatin C may partly reflect fat-mass reduction rather than GFR improvement.

How to Order and Monitor Cystatin C in Hormone-Therapy Patients

Baseline Assessment

Order cystatin C before initiating TRT, GLP-1 therapy, or HRT if baseline kidney function is uncertain or if creatinine-based eGFR is borderline (60 to 90 mL/min/1.73 m²). For premenopausal women, draw during the follicular phase (cycle days 2 to 10) to avoid the luteal rise.

Monitoring Intervals

• TRT: baseline, 3 months, 12 months, then annually if stable.
• HRT/OCP: baseline, 6 months, then annually.
• GLP-1 agonists (semaglutide, tirzepatide): baseline and 12 months; falling cystatin C may indicate both GFR improvement and adiposity reduction.

Action Thresholds

A single cystatin C value above 1.10 mg/L in an adult under 60 warrants repeat testing in 6 to 8 weeks under controlled conditions (follicular phase if applicable, euthyroid, no acute illness, no recent corticosteroids). Persistent elevation above 1.10 mg/L should prompt referral to nephrology or at minimum a measured GFR. Cystatin C above 1.30 mg/L in a patient under 50 with no confounders is consistent with CKD stage 3a or worse and should never be attributed to hormones alone without further evaluation.