Other causes for my tachycardia? all normal causes seem to be ruled out heart structure is normal, blood test shows normal ranges too, thyroid fine.

Depends: Depends upon the type of tachycardia - PSVT / A fib/flutter / sinus tach? PSVT is an electrical problem in the AV node of the heart. In a young male AFIB is iikely related to alcohol use and sinus tach is likely related to anxiety.

Good question: Often, anxiety can cause palpitations and tachycardia, even when you're not feeling stressed. Thyroid issues may contribute. It sounds like you have seen an internist or cardiologist for this. I would follow your doctor's recommendations.