95 % of hypertension is designated as primary HT .What does it mean ? It means 95 % of times we do not know what exactly is the cause for raised blood pressure . Simply stated . . . it reflects 95% ignorance .
So what is secondary hypertension ?
Secondary HT is the one, in which we have specific reason for the raised BP. The most important cause is Renal , endocrine etc.
When will you suspect renal HT ?
https://drsvenkatesan.wordpress.com/2010/09/01/when-will-you-suspect-reno-vascular-hypertension/
How is secondary HT different ?
- Occur at relatively at young age (<45)
- It is generally more severe.
- Diastolic BP is proportionately higher ,
- End organ damage is more.
- It is very unlikely primary HT to present as acute LVF. One rule of thumb is , if diastolic blo0d pressure is > 120 never diagnose primary HT . Some amount of renal component is very likely.
Is stress related HT a form of secondary HT ?
Dr.S.Venkatesan MD 
I think age no longer limits us from getting primary HTN due to our current lifestyle. I have seen a young patient at his 20s getting the diagnosis of DM and hypertension and no secondary causes have been found !!