How much does testosterone drop during contest prep?

Short answer

A lot. It fell 38% on average in 9 men, and cases that cut to 4-5% body fat dropped over 70%, below the normal range. Even so, muscle was mostly kept, and levels returned to baseline within 5-6 months after the show.

How much does testosterone drop during contest prep?
'Extreme dieting → testosterone hits rock bottom' — how true is it?
A common belief: by the end of contest prep, testosterone hits rock bottom. We checked with numbers how much it really drops in drug-free male bodybuilders, and what happened to muscle and recovery.
Only the dieters dropped
Those training without dieting stayed the same
13 men preparing for a contest were compared with 10 who were not dieting, from 23 weeks to 1 week before the show. Only the 13 dieters saw testosterone and its readily usable (free) form fall. The cause was the long diet, not the training.
9 men dropped 38% on average
From 16 weeks to 1 week out
In 9 male bodybuilders, testosterone fell from 16 weeks to 1 week before the contest, as if 100 became 62. The readily usable form fell from 100 to 51, close to half. They ate plenty of protein all along: about 196-217 g a day for a 70 kg (154 lb) person.
Cases that went all the way dropped over 70%
6-8 months, down to 4-5% body fat
1 man cut body fat from 14.8% to 4.5% over 6 months, and his testosterone went from 100 to 25. Another cut from 3,860 kcal to 1,724 kcal a day over 8 months and went from 100 to 28. In an analysis pooling case studies, all 6 men whose testosterone was measured fell below the normal range.
So did muscle go with it?
Testosterone fell 38%; lean mass fell 0.9 kg
In the 9-man study, fat dropped from 8.8 kg to 5.3 kg, and lean mass (muscle and everything except fat) fell 0.9 kg, from 71.8 kg to 70.9 kg, over the last 8 weeks. In the 13-man study, strength went down. Muscle was mostly kept, but strength dropped.
After the show, it came back
Levels returned to baseline
The 1 man who fell to 25 out of 100 bounced back to 107, slightly above where he started. The 13 men were back to baseline 23 weeks after the show. The man who fell to 28 out of 100 had mostly recovered 5 months after the show.
But two days of extra food didn't bring it back
Two days later it fell another 20.9%
1 man who prepped for 18 weeks reached the bottom edge of the normal range just before the show. After eating a bit more for two days, his thyroid hormone rose 18.5%, but testosterone fell another 20.9%, dropping below the normal range. Recovery took months, not days.
Bottom line: it drops 38-75% by the end
Muscle was mostly kept, and levels came back over months
Testosterone fell 38% on average in 9 men, and cases that cut to 4-5% body fat dropped over 70%, below the normal range. Even so, lean mass fell only 0.9 kg, and levels returned to baseline within 5-6 months after the show.