You cannot build a decent model with the given information. Still required are:
1) Time to maximum serum concentration, and
2) Absorption effectiveness.
I could not fine the time to maximum serum concnetration right off, but the absorption effectiveness of synthroid is an astoundingly broad 40%-80%! Maybe I'm just not looking, but that strikes me as a massively broad range. No wonder recommended administration requires a slow start. One would not want to assume 40% if you're going to get 80%.
Having said that:
Change in Concentration = Dosage*Effectiveness*(Function of Absorption Rate)*(Function of Clearance Rate)
So far, you have two of the four pieces for a basic model.