Florence Henderson Death Certificate

Florence Henderson secretly battled serious heart issues for 10 years before she died of a heart attack ... this according to her death certificate obtained by TMZ.

The 'Brady Bunch' star passed away at UCLA Medical Center in Santa Monica on Nov. 24th.

The death certificate notes she suffered from heart disease for 10 years.

5 years ago, Henderson was diagnosed with an irregular heartbeat.

The death certificate also notes Henderson was in the entertainment industry for 64 years ... legend.

R.I.P.