This study directly applied T3 to the nerves-- large sciatic nerves that were damaged. It was not delivered orally.

Oral administration will dilute your dose considerably.

It is well known that hypothyroidism causes peripheral nerve problems. This was my initial cause, in fact.

When I was given Synthroid (T4) finally, my nerves improved considerably.

The body normally converts T4 to T3 in tissues as it is needed.
If one takes too much T3 it will promote bone tear down and create osteoporosis. It can also stress the heart. This is why the body is careful in making small amounts as needed.