Thymulin is a zinc-dependent nonapeptide hormone produced by thymic epithelial cells. It is the only thymic hormone with a fully characterized chemical structure, and it requires zinc for biological activity. As thymic function declines with age, circulating thymulin levels drop — becoming virtually undetectable by age 60.
No thymulin formulation is FDA-approved. All protocols below are derived from published research and community experience. This is not medical advice.
Quick Reference: Standard Protocol
| Parameter |
Standard Protocol |
| Dose |
2 mg (20 units on insulin syringe) |
| Route |
Subcutaneous injection |
| Timing |
PM (evening) |
| Frequency |
Every day |
| Cycle |
20 days, 3 times per year |
| Vial size |
20 mg |
| Reconstitution |
2 mL bacteriostatic water → 10 mg/mL |
| Draw amount |
20 units on insulin syringe |
| Zinc co-administration |
15-30 mg elemental zinc daily (required) |
| Storage |
Refrigerate, use within 28 days |
Run 2 mg daily for 20 days, then stop. Repeat 3 times per year (roughly every 4 months). Zinc supplementation is non-negotiable — thymulin is biologically inactive without zinc. For the full thymulin profile, see our thymulin benefits and research guide.
Cycling Details
Unlike peptides that run 8-week cycles, thymulin uses shorter, concentrated 20-day bursts repeated 3 times annually. This provides focused immune support windows — the 20-day period is sufficient to promote T-cell maturation and immune recalibration.
Running it 3 times per year (roughly every 4 months) prevents overuse while maintaining immune benefits year-round. Short, concentrated cycles allow the immune system to consolidate T-cell maturation changes between courses.
Zinc is critical: Thymulin exists as inactive apo-thymulin (peptide alone) and active zinc-thymulin (Zn-FTS). Without zinc, thymulin cannot interact with its T-cell receptor (Dardenne et al., 1994). Supplement 15-30 mg/day elemental zinc (picolinate, glycinate, or citrate) before and during each cycle. Do not exceed 50 mg/day long-term — excessive zinc depletes copper.
Routes of Administration
Subcutaneous (standard and only route): Lower abdomen, upper outer thigh, or back of upper arm. Rotate injection sites. Use 29-31 gauge, 1/2 inch insulin syringe. Evening dosing aligns with sleep-associated immune system activity.
Reconstitution Quick Reference
| Vial Size |
BAC Water |
Concentration |
2 mg Dose |
| 20 mg |
2 mL |
10 mg/mL |
20 units |
20 mg vial + 2 mL BAC water = 10 mg/mL. Your 2 mg dose is 20 units on an insulin syringe. One vial lasts 10 days — you need 2 vials per 20-day cycle.
Swirl gently — do not shake. Refrigerate at 2-8°C and use within 28 days.
Where These Numbers Come From
Thymulin research spans several decades of immunology, though no large-scale clinical dosing trials exist comparable to thymosin alpha-1's dataset.
Zinc Dependency: Age-related thymulin decline is not solely due to reduced thymic production — it is partly driven by declining zinc availability in the thymic microenvironment (Mocchegiani et al., 1995). This is why zinc co-supplementation is considered essential.
Thymulin Structure: The 9-amino acid peptide (Glu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn) was fully characterized by Bach et al. (1989), establishing it as the only thymic hormone with a defined chemical structure (Bach et al., 1989).
Immune Function: Thymulin's primary role is T-cell differentiation and maturation at the thymic level. It acts upstream of thymosin alpha-1, which enhances mature T-cell function. The 20-day cycle length is empirically derived from the time course of T-cell maturation processes.
Stacking Protocols
Thymulin + Thymosin Alpha-1
| Peptide |
Dose |
Route |
Timing |
Purpose |
| Thymulin |
2 mg daily |
SC |
PM |
T-cell maturation (upstream, development) |
| Thymosin Alpha-1 |
1.5 mg 5on/2off |
SC |
AM |
T-cell activation (downstream, function) |
Complementary mechanisms — thymulin develops T-cells, TA-1 activates them.
Thymulin + LL-37
| Peptide |
Dose |
Route |
Timing |
Purpose |
| Thymulin |
2 mg daily |
SC |
PM |
Adaptive immunity (T-cell arm) |
| LL-37 |
50-100 mcg |
SC |
AM |
Innate immunity (antimicrobial) |
Different arms of the immune system — adaptive vs innate.
Side Effects & Safety
- Injection site reactions — mild redness, pain, or swelling (transient)
- Mild fatigue — occasionally in the first few days
- Mild headache — infrequent
- Transient flu-like symptoms — rare, possibly indicating immune engagement
- No serious adverse events reported in thymulin research literature
- Zinc-related GI discomfort — nausea from zinc supplementation, take with food
- Contraindicated in active autoimmune disease, organ transplant recipients, and lymphoproliferative disorders
Frequently Asked Questions
What is the standard thymulin dose?
2 mg subcutaneous daily in the evening for 20 consecutive days, repeated 3 times per year. Draw 20 units from a 20 mg vial reconstituted with 2 mL BAC water. Zinc supplementation (15-30 mg/day) is required.
Does thymulin need zinc to work?
Yes — thymulin is biologically inactive without zinc. The active form is zinc-thymulin (Zn-FTS). Always ensure adequate zinc status before and during use.
How long should a thymulin cycle last?
20 consecutive days, repeated 3 times per year (roughly every 4 months). Shorter, concentrated cycles provide focused immune support without continuous exposure.
Can thymulin be combined with other immune peptides?
Yes — commonly stacked with thymosin alpha-1 (complementary T-cell mechanisms) or LL-37 (innate antimicrobial defense). No clinical data exists on these combinations.
What are the side effects of thymulin?
Remarkably clean profile. Mild injection site reactions, occasional fatigue, rarely headache. No serious adverse events reported. Zinc co-supplementation may cause GI discomfort — take with food.
For educational and research purposes only. This is not medical advice. Thymulin is not FDA-approved for any indication.