Thank you for your question.
50 micrograms is one of the lowest doses of Synthroid. So, doubling it to 100 micrograms for only one day would not necessarily cause a problem, but it could. If you have not had a problem yet, then you likely will not have any problems. But, be sure to look out for continuous fast heart rate, shaking, tremors, an overall anxious feeling, or unexplained diarrhea. If any of these symptoms occur and are continuous in nature then you may be slightly overstimulated from the thyroid hormone (Synthroid). If this does happen, be sure to contact your doctor right away. If the symptoms do occur, since you only took an extra amount once, it would only be transient in nature.
You may see symptoms within the next day or so. You could skip your dose tomorrow to make sure that everything is ok, but check with your doctor if you have doubts. I have many patients on much larger doses than 50 mcg of Synthroid without any problems, so the odds of you having a problem from taking 100 mcg just once are going to be slim. But be sure to watch out for the symptoms. Likely, you will be just fine. Thyroid hormone is extremely safe to use and take.
Again, if anything seems off for too long, contact your doctor.