Overview
Hormone optimization with peptides differs fundamentally from hormone replacement: secretagogue peptides stimulate endogenous production rather than replacing the hormone directly. This preserves pulsatility, maintains axis feedback, and avoids the suppressive effects of exogenous hormones. See the full guide Peptides vs. Hormones for the detailed mechanistic distinction.
Growth Hormone Axis
The most studied hormonal application for peptides. GH secretion naturally declines ~14% per decade after age 30 (somatopause). Restoring physiological GH pulsatility through secretagogues has documented effects on body composition, bone density, and quality of life in GH-deficient adults.
Key compounds:
| Compound | Mechanism | Half-life | Dosing |
|---|---|---|---|
| Sermorelin | GHRH 1-29 fragment → GHRH-R | ~12 min | Daily SubQ before bed |
| CJC-1295 w/o DAC | GHRH analog → GHRH-R | ~30 min | 1-3x daily |
| CJC-1295 with DAC | GHRH + albumin binding | 6-8 days | Weekly |
| Ipamorelin | GHSR-1a agonist | ~2 hours | 1-3x daily |
| GHRP-2 | GHSR-1a agonist (potent) | ~2 hours | 3x daily |
| MK-677 (Ibutamoren) | Oral GHSR agonist | ~24 hours | Daily oral |
CJC-1295 + Ipamorelin combination: Targets complementary receptors (GHRH-R and GHSR-1a), producing synergistic GH release exceeding either agent alone. The most common GH secretagogue stack in research protocols.
Testosterone Axis — Peptide Adjuncts
Peptides are not testosterone; they don’t replace it. However, several peptides are used in the context of testosterone optimization:
Gonadorelin (GnRH): Stimulates LH and FSH release, maintaining testicular function on TRT. FDA-approved (Factrel); also compounded.
hCG: Peptide hormone (LH analog); stimulates testicular testosterone production. Used to maintain fertility and testicular size on TRT. Compounded availability significantly restricted since 2020 (biologic reclassification).
PT-141 (Bremelanotide): Addresses one symptom of testosterone deficiency (low libido) through a central melanocortin mechanism, without directly modifying testosterone levels.
Thyroid Axis
No research peptides with strong evidence specifically modulate thyroid hormone levels. Thyroid hormone optimization remains the domain of conventional medicine (T4/T3 replacement). Some GH elevation may indirectly affect T4→T3 conversion through hepatic effects — but this is a secondary consideration.
Evidence-Graded Approach
The strongest evidence in this category is for:
- GH secretagogues in diagnosed GH deficiency (human Phase I/II data)
- Gonadorelin and hCG for gonadotropin supplementation (FDA-approved indications)
- PT-141 for HSDD (FDA approved Phase 3 data)
Anti-aging optimization in eugonadal, GH-sufficient individuals has a weaker evidence base — effects exist but are smaller and individual responses vary considerably.