Worrying is a natural part of being human. Let's not forget that we evolved from primates who needed to worry about predators while carefully traversing the landscape, and then from cave men who also had the same concerns - perhaps even more so given their higher intelligence and understanding.
A scientific study that looked at 26 people with a severe anxiety disorder and 18 'healthy' volunteers, found that while the normal volunteers were generally unaffected, the anxiety sufferers who worried the most and overthought tended to have higher intelligence and a higher IQ score.
The study concluded that there is indeed a link between anxiety and higher intelligence.
Brain scans conducted by another scientist found that the white matter responsible for critical communications between brain cells, also responded to both worry and high intelligence in a similar fashion.
As with many mental conditions, people with anxiety can suffer from some or all symptoms with varying severity. Some may only be affected at certain times or during particular situations, and to an extent where there may be no outward signs that others will be able to pick up on. More severe users may find themselves debilitated and unable to function during episodes, with erratic breathing, uncontrollable shaking, and thoughts filled with dread. This is what separates naturally-evolved anxiety sufferers, which could indicate higher intelligence, from those who are literally mentally and, in turn, physically disabled by it.