Difference between 7-keto DHEA and DHEA?

[mt]

I know that 7-keto is used more to rev up the thyroid and help you lose weight but are they still the same basic hormone? Specifically, if I am over 40 years old and just want to make sure I have an adequate supply of DHEA in my system and I'm already taking 100mg/day of 7-keto is it still necessary to take the 25mg/day of the DHEA I'm already using?