In a nutshell NO. T4 is a storage hormone also known as a
"pro-hormone" who's sole function is the building block that T3, the
bio-active hormone, is made from. There are people who have been on T3 only for 20 years
without ill effects. I have been on T3 only for nearly 4 years and don't miss T4 at all. (updated 12/12).
It is VERY important once you are on T3 to stay on it or change very gradually onto
something else. If you suddenly stopped taking T3 (for over a week with no replacement
other wise) you could go into myxedema coma
THEORETICALLY, though we have never heard of this happening. It's well worth
carrying a medical warning note that you are on T3 and the dosage in case you
are rendered unconscious in a car accident. Most people that still have a thyroid
would quickly get some T4 and thus T3 from their own thyroid but a thyroidless or
otherwise totally dependent person should never stop T3 treatment even for a day.