Did you wear the monitor for just 24 hrs? Sometimes wearing the monitor for a week at a time will show things that a 24 hr. test will miss.
Some types of cardiac arrhythmia can occur relatively infrequently.
There are many tests that can be done to eval thyroid function. Not just one -- though many docs would have us believe that the TSH thyroid screening test tells all. So if you suspect a possible thyroid cause, gather up your hard copies of thyroid related test results, figure out which tests haven't been done, and request them.
Cortisol (adrenal hormone) deficits can interact with thyroid hormone and lead to heart palps -- esp. since adrenal hormones also influence electrolyte balance, which is so critical for cardiac function.
If you haven't had a recent draw for 8 a.m. fasting cortisol & ACTH level plus electrolytes, those are simple screens that might give indication if you have severe adrenal malfunction.
On the other hand, if the problem were something like a thyroid nodule that occasionally acts up & squirts out excess thyroid hormone, the best place to begin might be thyroid ultrasound.
Best wishes.