The manufacturer advises
waiting 30-60 minutes after taking your Synthroid before eating, and the official prescribing information
similarly makes reference to the decreased abosorption of Synthroid with dietary fiber. They make no specific recommendation for a timing related to fiber.
A review of the medical literature on the subject reveals a study done in 1998
This study came to the conclusion: In summary, neither calcium polycarbophil nor psyllium hydrophilic
mucilloid are likely to cause malabsorption of LT4 that could be
detected by these methods
A study done in 1996 looked at fiber enriched diets and Synthroid
This study concluded: These results indicate a decrease in T4 bioavailability by dietary
fiber through a mechanism involving nonspecific adsorption of T4 to
dietary fibers. Increased intake of dietary fiber may account for the
need for larger than expected doses of T4 in some hypothyroid patients.
These two studies are somewhat contradictory in that the one specifically looked at fiber supplements and their effect on absorption and found no significant effect, while the other looked at diet and found a significant effect.
If you want to extend your food free interval to 60 minutes, that may be reasonable, however, as long as your daily routine is STABLE, and you don't make dramatic changes day to day (or month to month - i.e. oatmeal in the winter, and fruit and toast in the summer) - any effect will be stable.
Assume that you reduce your absorption by 50% - as long as you don't change anything (you take the same dose, eat the same time, and eat the same thing nearly every day) - then your absorption will stay at 50%. If your TSH/Free T4 are OK and in the normal range on that regimen, it doesn't really matter that the dose you have to take is 50% higher than if you took a different diet/timing of your medication.