If I have thread veins, does this mean there is an underlying cause?

Not necessarily. It is, however, very important to check for any underlying causes, because treatment of thread veins will not be very successful if the underlying causes are not treated first (i.e. the treated thread veins would be extremely likely to return).

A colour duplex ultrasound scan would be required to determine whether you have any faulty valves, and only then would we be able to give you an accurate proposed treatment plan.