This is interesting - check it out. A study that shows patients with a low Body Mass index ( BMI) of less than 24 tend to get under dosed whereas patients with a high BMI ( i.e. overweight) tend to get overdosed by doctors prescribing T4 or Na Thyroxine. Historically doctors guestimated how much thyroxine to prescribe at 1.6-1.7 micrograms/kg/day and adjusted based on sequential blood results until your blood tests results confimed euthyroidism. This study was based in a patient population following thyroidectomy but I think that if you have been on thyroxine for a long time for Hashimoto;s disease you can pretty well assume that your residual thyroid gland has very little remaining activity.