The late, great Spike Milligan was repeatedly hospitalised on mental wards because of his bi-polar (or manic depression as it was then termed).
I have had two cousins (both male) who committed suicide. One hanged himself in 1973, after a few failed attempts, the other drowned himself in the 90s. IMHO, anyone who takes their own life, and has not been diagnosed with a fatal illness, must be suffering some sort of mental turmoil. Surely?
And, like Gary Speed, some are very successful at hiding their condition from their nearest and dearest.