Andrew Stern -- the husband of "Deal or No Deal" showcase model Katie Cleary who committed suicide Sunday -- went on a downward spiral after undergoing hormone replacement therapy for his libido ... TMZ has learned.
TMZ broke the story ... Stern shot himself in the head at a San Fernando Valley firing range. Sources say he was deeply depressed, largely over the meltdown of his marriage.
Close friends of Andrew tell us ... 2 years ago he went to a Beverly Hills health and wellness center because he was feeling lethargic and had a waning libido. We're told the center gave him a kit, which included human growth hormones and testosterone. Both were injectables.
The friends say he started experiencing drastic mood swings and became extremely irritable. Andrew kept the regimen up for a year but couldn't take it anymore and stopped cold turkey.
We're told abruptly stopping such therapy is extremely dangerous and in Andrew's case he spiraled into a deep depression, which got much worse because his business failed and everything then took a toll on his marriage.
The friends tells us ... Andrew -- who filed for divorce in April -- went nuclear after seeing pics of his wife in Cannes, nuzzling up to Leo DiCaprio. Katie's rep says the pics didn't have an impact on Andrew, but his friends scoff and say it had a HUGE affect.
One friend tells us he spoke with Andrew the day before he killed himself -- Andrew said he was having "a tough day" but assured him he'd be OK.