For licensed clinicians · Educational reference

Clinician tools: reference calculators with the math exposed.

Four reference calculators for endocrinology, GLP-1 therapy, TRT, and longevity. Each shows the underlying equation and primary citation. Built by OPTML for licensed clinicians, residents, and pharmacists, free to use, no account required.

Updated 2026-04-29 · Reviewed by OPTML Clinical Team · For educational use; not a substitute for clinical judgment.

Endocrinology
Vermeulen Free Testosterone
Total T + SHBG + Albumin → Free T, Bioavailable T
Longevity
Levine PhenoAge
9 lab markers → biological age + delta
GLP-1 Therapy
GLP-1 Titration Assistant
Drug, week, tolerance → recommended next dose
Hormones
TRT Dose Calibration
Current T + target → testosterone cypionate weekly dose
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Educational reference for licensed clinicians. These calculators implement published equations from peer-reviewed literature and product labeling. They do not establish a physician-patient relationship, do not constitute medical advice, and are not intended for direct patient diagnostic or therapeutic decisions outside of clinical judgment. Verify all outputs against your patient's full clinical picture.
Endocrinology · Mass-Action Equation

Vermeulen Free & Bioavailable Testosterone Calculator

Computes free and bioavailable testosterone from total T, SHBG, and albumin using the Vermeulen mass-action equation. Source: Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. JCEM 1999;84(10):3666-3672. Affinity constants: KSHBG = 1.0 × 10⁹ L/mol, KAlb = 3.6 × 10⁴ L/mol.

Inputs

ng/dL
nmol/L
g/dL
Free testosterone
ng/dL
Free T (pg/mL)
Free T (pmol/L)
Bioavailable T
% Bioavailable
Adult male reference (Endocrine Society 2018): Total T 264-916 ng/dL; Free T 9.3-26.5 ng/dL (0.32-0.92 nmol/L) via mass-action calculation. Hypogonadism workup typically initiated when total T < 300 ng/dL on two morning samples with symptoms.
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Vermeulen mass-action equation: Solves the quadratic KT·N·[FT]² + (N + KT(SHBG − TT))·[FT] − TT = 0 where N = KAlb·[Alb] + 1.

Constants: KSHBG = 1.0 × 10⁹ L/mol, KAlb = 3.6 × 10⁴ L/mol. Albumin converted from g/dL to mol/L via MW 66,500 g/mol.

Bioavailable T = Free T × N (i.e., free + albumin-bound). Validated against equilibrium dialysis (gold standard) with R² ≈ 0.94 in the original 1999 publication.

Longevity · Phenotypic Age

Levine Phenotypic Age (PhenoAge) Calculator

Computes biological age from 9 standard lab markers using Levine's phenotypic age formula, derived from NHANES III mortality outcomes. Source: Levine ME et al. An epigenetic biomarker of aging for lifespan and healthspan. Aging (Albany NY) 2018;10(4):573-591. Phenotypic age > chronological age predicts increased all-cause mortality independent of chronological age.

Inputs (9 markers + age)

yr
g/L
µmol/L
mmol/L
mg/L
%
fL
%
U/L
10³/µL
Phenotypic age
yrs
Mortality score
DNAm GrimAge proxy
xβ (linear predictor)
Δ vs chronological
Interpretation: ΔPhenoAge > 0 (biological older than chronological) tracks with increased all-cause mortality and incident chronic disease. Each +1 year of ΔPhenoAge corresponds to a hazard ratio ≈1.045 for all-cause mortality (Levine 2018, Table 3). Use serially to track intervention impact.
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Linear predictor (xβ):

xβ = −19.9067 + 0.0336·Alb − 0.0095·Cre + 0.1953·Glu + 0.0954·ln(CRP) − 0.0120·Lym + 0.0268·MCV + 0.3306·RDW + 0.00188·AlkPhos + 0.0554·WBC + 0.0804·Age

Mortality score: M = 1 − exp(−exp(xβ) · (exp(0.0076927·120) − 1) / 0.0076927)

PhenoAge (years): 141.50 + ln(−0.00553 · ln(1 − M)) / 0.09165

Units must match Levine 2018: Albumin g/L (NOT g/dL, multiply by 10), Creatinine µmol/L (NOT mg/dL, multiply by 88.4), Glucose mmol/L (NOT mg/dL, divide by 18.018), Alk Phos U/L, WBC 10³/µL, MCV fL, RDW %, Lymphocyte %, CRP mg/L (log-transformed).

GLP-1 Therapy · Dose Titration

GLP-1 Titration Assistant (Semaglutide / Tirzepatide)

Recommends next dose based on current week, current dose, and tolerance, mapped to the FDA-labeled escalation schedules. Sources: Wegovy (semaglutide) FDA Prescribing Information; Zepbound (tirzepatide) FDA Prescribing Information; clinical trial protocols STEP-1 (NEJM 2021) and SURMOUNT-1 (NEJM 2022).

Inputs

wk
mg/wk
Recommended next dose
mg/wk
Action
Re-evaluate
Show the schedule

Semaglutide standard escalation (every 4 weeks if tolerated): 0.25 → 0.5 → 1.0 → 1.7 → 2.4 mg/wk. Maintenance: 2.4 mg/wk (or step down to 1.7 mg if 2.4 not tolerated).

Tirzepatide standard escalation (every 4 weeks if tolerated): 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg/wk. Maintenance options: 5, 10, or 15 mg/wk depending on weight loss target and tolerance.

If significant GI symptoms: hold current dose for additional 4 weeks before next escalation. Counsel hydration, smaller meals, avoid high-fat. Consider antiemetic if symptoms persist.

Note: compounded preparations may use the same active ingredient at the same titrations; provider judgment applies for off-label indication or dose individualization.

Hormones · TRT Dose Tuning

TRT Dose Calibration (Testosterone Cypionate)

Estimates testosterone cypionate weekly dose adjustment to reach a target total testosterone, using a population-average dose-response coefficient. Sources: Bhasin S et al. Endocrine Society Clinical Practice Guideline. JCEM 2018;103(5):1715-1744; AUA Testosterone Deficiency Guideline 2018. Population dose-response: ~10 ng/dL trough total T elevation per 1 mg/wk testosterone cypionate, modulated by SHBG.

Inputs

ng/dL
mg/wk
ng/dL
nmol/L
Recommended weekly dose
mg/wk
Δ from current
Expected new T
SHBG modifier
Re-test labs6-8 weeks
Safety bounds: Suggest monitoring hematocrit (cap at 54%), estradiol (sensitive assay), and PSA (men > 40) at 6-8 weeks post change. Doses > 200 mg/wk warrant specialist review. AUA target trough is generally 450-600 ng/dL; Endocrine Society target is mid-normal range (~500-700 ng/dL). Adjust target to your patient's clinical picture.
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Base dose-response: ΔT (ng/dL) ≈ Δdose (mg/wk) × 10

SHBG modifier (this tool): If SHBG > 50 nmol/L, dose response is dampened (~0.85×); if SHBG < 20 nmol/L, dose response is amplified (~1.15×). This is heuristic, individual variation is substantial.

Recommended dose: Dosenew = Dosecur + (Ttarget − Tcur) / (10 × SHBG modifier), capped at 200 mg/wk without specialist input.

This is a population estimate; verify against the patient's labs at 6-8 weeks post adjustment. Some patients require split dosing (2× weekly) to flatten peak/trough; others tolerate every-2-week dosing well.

For licensed clinicians and trainees

These tools are free, no account required, no data stored server-side. Calculations happen entirely in your browser. Refer them to colleagues, link from your residency program wiki, or bookmark for chart-side use.

Suggestions for additional reference tools? Email the OPTML clinical team, we'll consider building it.