There's a long held belief that this is true. I first heard it from an old sage who said that when the thyrotoxic patient develops atrial fib, “run for the heparin!”
Thyrotoxicosis is not listed as one of the CHADs risks, so fact or myth?
All the evidence is examined in this review in the Texas Heart Institute Journal. It turns out that multiple studies show an alarmingly high rate of thromboembolism in thyrotoxic patients with a fib compared to rates reported in non-thyrotoxic patients. But these studies were small and had methodologic problems. At least one larger study questioned the association. A systematic comparison of thyrotoxic and non-thyrotoxic patients with a fib has not been done.
Guidelines differ. The AHA/ACC guidelines suggest that indeed thyrotoxicosis is a risk factor and is itself a reason for anticoagulation. The authors of this review conclude:
Given the lack of clear evidence, the ACC/AHA classification of thyrotoxicosis as a moderate thromboembolic risk factor seems to be reasonable, and the recommendation to initiate anticoagulation when there are no contraindications appears to be warranted. More evidence-based trials are necessary to clarify this issue.