Assessment of optimal L-thyroxine replacement dose by the TRH test.

Two hundred patients taking varying L-thyroxine replacement doses were studied using a normal TRH test as the index of optimal replacement dose. The mean optimal dose was 141 microgram/day. Normal serum T3 and FT3I were found in most patients, whatever the TRH response, and they are probably too unspecific. Serum T4 and FT4I were elevated in many patients with a normal TRH response. A higher range for FT4I of 102-166, although only 66.5% accurate, gave the best index of optimal L-thyroxine replacement of the single in-vitro tests.