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 μg/day. Normal serum T8 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.
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