I strongly disagree with the manufacturer conversion charts (as do many others). I felt horrible on an “equivalent” dose of NDT when switching from Synthroid once most of the Synthroid had left my system (somewhere around the 3 week mark). Since then I’ve been self-dosing and have titrated my dose up to 4 grains. I don’t know that I’ll need to stay on this high of a dose, but I’m taking Prilosec at the moment and this is most likely decreasing my absorption due to reduced stomach acid. I believe that most people that don’t feel well on NDT simply aren’t on a high enough dose. For others, NDT simply may not work for them.
Note that 2-4 grains is the typical NDT dose range for most patients according to MULTIPLE docs whose opinions I respect including: Dr. John Lowe, Dr. Henry Lindner, Dr. John A. Robinson, Dr. Dean Ward, Dr. Mark Starr, etc. with the "average" dose probably being somewhere around 3-3.5 grains (Dr. Mark Starr quoted this amount in his book).
I have a Word doc with hundreds of quotes from different docs based on hundreds (if not thousands) of hours of research I've done over the years and all of these docs state that 2-4 grains is the average dose range that their patients end up on when dosing by symptom relief (not blood tests) and watching for irregular pulse, blood pressure, basal temperature elevation, etc. Some need less, some need more.
To the person who mentioned TiredThyroid.com - I’ve read Barb’s book and I think she did a great job with her research. One area where I’m not sure I agree with her however is that we should be trying to replicate physiological (normal) doses of what they human thyroid would make. She also suggested that thyroid labs should be taken 6-8 hours after a dose and I strongly disagree with this. You will always appear to be overmedicated if taking blood 6-8 hours after a dose of NDT or straight T3. If you wait 24 hours to test blood after a dose of NDT, you will get an accurate reflection of the FT3 level in your blood without a falsely elevated result. 2 different studies I've read have confirmed that it takes 18-24 hours for T3 levels to decrease to "normal" after dosing with anything containing T3. For example, a member on this forum stated that they always wait 24 hours after a dose of NDT for blood work. On 2 grains, their FT3 was around mid-range. On 3 grains, their FT3 was near the top of the range after 24 hours. The point being that even though T3 is processed quickly by the body, the levels will still increase over time - even after the last dose is out of your system.
As for trying to replicate physiological doses of thyroid hormone, I'm not sure this works in practice. For example, the average human thyroid might make 80-110 mcg of T4 in a day and 6-10 mcg of T3 per day (the human body likely makes somewhere around 30 mcg of T3 in a day, but most of it is from conversion of T4 and not made directly by the thyroid). Yes, in an IDEAL world where you could take a slow release pill that released a steady amount of hormone over 24 hours or have a slow drip IV infusion of thyroid meds, replacing what the body would make on its own makes perfect sense - in theory. However, when taking thyroid hormone orally once or twice a day, this is in no way similar to what the body does when it makes thyroid hormone a little bit at a time, 24 hours a day. In addition, taking thyroid hormone orally will sharply decrease TSH when the body senses a sudden influx of meds (particularly T3 containing meds) which in turn decreases T4>T3 conversion (TSH helps with this conversion). Therefore it’s sometimes necessary for us to take unnaturally high doses of thyroid medication (i.e. 4 grains of NDT = 152 mcg T4 and 36 mcg T3) to achieve symptom relief.
Also keep in mind that the body doesn’t absorb everything you take in a pill form. Absorption rates vary dramatically. Some people might only absorption 50% of what they ingest orally where as others might absorb much more.