What the studies show after a single injection
1) Testosterone enanthate 100 mg or 200 mg, single IM injection
A classic Fertility & Sterility study measuring serial hormones after one injection reports rapid gonadotropin drops:
- Quote: “Serum LH and FSH fell rapidly… The first decrement occurred between days 1 and 2 (−39 ± 6% for LH and −28 ± 9% for FSH).”
PubMed:
Comparison of the kinetics of injectable testosterone in eugonadal and hypogonadal men - PubMed
2) Testosterone enanthate single-dose study showing larger suppression by day 4
Healthy volunteers given
one IM dose of testosterone enanthate had large LH and FSH suppression by day 4, even at a dose that is not extreme:
- Quote: “S-LH and s-FSH were significantly decreased… on Days 4 and 14.”
- Quote with numbers (single dose 125 mg): “After the 125 mg dose, LH decreased by 65%… and… FSH 45%” (Day 4).
- Quote with numbers (single dose 500 mg): “73% and 92% decrease in LH on Days 4 and 14” after 500 mg.
Full text:
Effects of different doses of testosterone on gonadotropins, 25-hydrox | SAR
3) Testosterone cypionate: LH suppression after the first injection
Another Fertility & Sterility paper (testosterone cypionate) states suppression occurs even with the first injection:
- Quote: “Luteinizing hormone became suppressed in men receiving their first intramuscular TC injection…”
PubMed:
Hormone kinetics after intramuscular testosterone cypionate - PubMed
What this means in plain terms
- After one injection, it is normal to see roughly 25% to 65% LH suppression within the first few days depending on dose and design, with some studies showing larger suppression by day 4.
- That is “shutdown” in the only sense that matters physiologically: the brain-pituitary signal (LH/FSH) drops, so testicular endogenous production drops.