I'm a dork!
I found this for starters:
"Levothyroxine, also known as L-thyroxine, synthetic T4 or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine (thyroid hormone). The natural hormone is chemically in the L-form, as is the pharmaceutical agent. Dextrothyroxine (D-thyroxine) briefly saw research as an anticholesterol agent but was pulled due to cardiac side-effects"
Both are in the same chemical form, so is it an isomer?