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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.
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. the clinical literature 1999;84(10):3666-3672. Affinity constants: KSHBG = 1.0 × 10⁹ L/mol, KAlb = 3.6 × 10⁴ L/mol.
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.
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.
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).
Visualizes common gradual dose-escalation patterns used in physician-supervised GLP-1 protocols, where the dose is increased over weeks to support tolerance. References: Drucker DJ. Cell Metabolism. 2018;27(4):740-756 (GLP-1 receptor biology and dose-response physiology); Drucker, Habener & Holst. J Clin Invest. 2017;127(12):4217-4227 (GLP-1 receptor pharmacology).
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 standard active pharmaceutical ingredient and titrations; provider judgment applies for off-label indication or dose individualization.
Estimates testosterone cypionate weekly dose adjustment to reach a target total testosterone, using a population-average dose-response coefficient. Sources: published research. published clinical practice guidelines. the clinical literature 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.
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.
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.
OPTML offers compounded semaglutide and tirzepatide, prescribed and overseen by U.S.-licensed physicians. Compounded drug products are not approved or evaluated for safety, effectiveness, or quality by the FDA. OPTML is not associated with, endorsed by, or affiliated with the manufacturers of FDA-approved branded weight-loss medications, and does not sell or supply those branded products. Anyone interested in an FDA-approved branded medication should consult a licensed healthcare provider or pharmacist. Individual results vary; no specific result is promised, and weight management requires ongoing medical guidance.
The whole picture in one sitting: who it suits, what the labs decide, what the first year actually looks like. Every clinical claim is cited.
How to protect lean mass while losing weight on a GLP-1.
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