Genryzon Igf1 Titration

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name: genryzon-igf1-titration
description: Titrate Genryzon (somatrogon, weekly long-acting growth hormone) dose in a paediatric GHD patient using serum IGF-1 SDS. Encodes the mandatory 4-days-post-dose sampling rule, the target SDS window, the 15% dose-reduction trigger, and the oral estrogen adjustment. Use when a clinician asks how to titrate somatrogon, when to sample IGF-1 on Genryzon, target IGF-1 SDS on weekly GH, what to do if IGF-1 is too high on somatrogon, IGF-1 rising on Genryzon, how to adjust GH dose in a girl started on oral OCP, or presents a paediatric GHD case on somatrogon with an IGF-1 value to interpret. Grounded in the Pfizer India Product Monograph (Genryzon LPD, 2022 — PfLEET 2022-0081166).
---

# Genryzon IGF-1 Titration

Adjust the weekly somatrogon dose using serum IGF-1 SDS. **Sampling timing is the single most-often-wrong step** — get that right first, then apply the SDS rule.

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## Step 1 — Draw the IGF-1 sample at the correct time

**Always draw the IGF-1 sample exactly 4 days after the prior weekly dose.**

Somatrogon has a pharmacokinetic profile such that IGF-1 peaks a few days post-dose and then falls. A sample taken 1 day post-dose overestimates; a sample taken 6–7 days post-dose underestimates. Only day-4 samples are interpretable against the reference range used by the monograph.

If the timing was wrong, **repeat the sample at day 4** before titrating.

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## Step 2 — Compare against the age- and sex-specific reference

**Target: IGF-1 SDS between −2 and +2** (aim close to 0 SDS).

Use the assay's own reference values for **age AND sex**. IGF-1 rises through puberty — a boy Tanner 4 has a much higher normal than a boy Tanner 1 of the same chronological age.

| IGF-1 SDS | Interpretation |
|---|---|
| **> +2** | Above target — **reduce dose by 15%** (see Step 3) |
| **−2 to +2 (ideal near 0)** | On target — no change; continue current dose |
| **Below target with poor growth velocity** | Under-dosed OR check adherence, injection technique, thyroid function, glucocorticoid interference; consider dose escalation only after ruling those out |
| **Below target with adequate growth velocity** | May still be acceptable — do not over-treat to raise IGF-1 if the child is growing well |

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## Step 3 — Reduce dose by 15% when IGF-1 SDS > +2

**If IGF-1 SDS exceeds +2:**

1. Calculate the current weekly dose (mg)
2. Reduce by 15% (new dose = current dose × 0.85)
3. Round to the nearest pen increment:
   - 24 mg pen → 0.2 mg increments
   - 60 mg pen → 0.5 mg increments
4. Re-check IGF-1 at the next planned visit (or sooner if clinically indicated), 4 days post-dose

**More than one 15% reduction may be required** — do not attempt a larger single reduction. Iterate.

**Worked example:**
Weekly dose 24 mg → 24 × 0.85 = 20.4 mg → round to nearest 0.5 mg → **20.5 mg once weekly.** Recheck IGF-1 (day 4 post-dose) at next visit. If still >+2, reduce again to ~17.5 mg.

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## Step 4 — Adjust for oral estrogen

**Oral estrogen (including combined OCPs, HRT) reduces the IGF-1 response to growth hormone** via first-pass hepatic effects on IGF-1 generation.

**When a female patient starts oral estrogen:**
- Re-check IGF-1 4 days post-dose after 6–8 weeks
- A **higher somatrogon dose** may be needed to keep IGF-1 in target range

**When a female patient stops oral estrogen:**
- Re-check IGF-1 similarly
- Dose may need to **decrease** to avoid crossing the +2 SDS threshold

**Transdermal estrogen** does not have the same first-pass effect — dose adjustment usually not required. If switching a patient from oral to transdermal, expect the somatrogon requirement to fall.

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## Step 5 — Rule out confounders before adjusting dose

Before treating an out-of-range IGF-1 as a dosing problem, exclude:

- **Wrong sampling timing** (not day 4 post-dose) — repeat
- **Adherence issues** (missed doses, wrong technique) — patient interview + demonstration
- **Assay switch** (different lab → different reference) — check reference range used
- **Concurrent illness / acute inflammation** — IGF-1 falls acutely
- **Untreated hypothyroidism** — blunts response → falsely low IGF-1
- **Concurrent glucocorticoid** (esp. ↑ dose) — inhibits GH effect → falsely low IGF-1
- **Malnutrition or protein deficiency** — falsely low IGF-1
- **Puberty stage shift** — the reference range changes; re-plot against current Tanner/bone age
- **Started/stopped oral estrogen** (Step 4)

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## Step 6 — Follow-up cadence

- Routine IGF-1 SDS review at **6–12 month intervals** (more frequent during puberty)
- **After every dose change**, re-check IGF-1 at the next visit (minimum 8–12 weeks) — day 4 post-dose
- Track auxological outcomes in parallel: **height velocity, weight, bone age, Tanner stage**. IGF-1 is a tool, not the goal — growth velocity and eventual final height are

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## Guardrails

- **Day-4-post-dose IGF-1 is non-negotiable** — a sample drawn on any other day cannot be interpreted against the monograph target
- **Never react to a single high IGF-1 without confirming timing was correct** and without checking for acute confounders (illness, glucocorticoid pulse, recent illness)
- **Do not exceed a 15% single-step reduction** — the monograph specifies iterative 15% steps
- **IGF-1 above +2 SDS is a safety trigger, not a cosmetic one** — chronically supra-physiologic IGF-1 is associated with growth of subclinical neoplasms and other risks; treat the number seriously
- **Do not raise dose above 0.66 mg/kg/week for a "faster growth" request** — the monograph anchors dosing to weight, not to parental expectation
- **Female patient started on OCP** is a routine trigger for re-checking IGF-1 — flag this actively in every teenage girl's visit
- **A single day-4 IGF-1 below −2 SDS in a growing child** is not automatically an under-dose — re-verify against pubertal-stage-matched reference and confirm growth velocity is genuinely poor before increasing

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## Source

Pfizer Products India Pvt. Ltd. **GENRYZON® (Somatrogon) Solution for Injection in Pre-filled Pen — Prescribing Information (India).** LPD version 2022-0081166 (PfLEET 2022-0081166). Section 4.2 "Dose titration" and Section 4.5 "Drug Interactions — Oral oestrogen therapy".