We know that 'lack of Synthroid' is NOT the cause of any thyroid problem. Exposure to chemicals, e.g. perchlorate, common food intolerances (e.g. wheat/dairy), adrenal fatigue and nutrient deficiencies are the cause of 'hypothyroidism' in the vast majority of cases.
An F3 felony is a shorthand expression for a class or category 3 (or III) felony in jurisdictions that use numbered offense categories. What such an offense is punishable by and whether it is punishable by more or less than an F2 felony depends completely on the laws of the jurisdiction in question.
If you were charged with theft in the third degree (sometimes referred to as Theft III) or theft in the second degree (Theft II), you are charged with a misdemeanor crime. If you were charged with theft in the first degree (Theft I) or aggravated theft in the first degree, then you are facing a felony charge. There are other related charges such as identity theft; forgery; criminal possession of a forged instrument; etc. which are outside the scope of the discussion here.