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.
In some states, each class of felony is assigned a number, with a Class 1 felony being the most serious crime, and a higher number class being the least serious crime. Likewise, some states use a letter system to designate different classes of felonies.