Evidence for Peripheral Autoregulation of Thyroxine Conversion
The reciprocal changes of triiodothyronine (T3) serum concentrations and (SC) and thyroxine (T4) SC observed in patients with varying degrees of primary hypothyroidism are well known. T3 SC are often still within normal limits, when T4 SC have already reached low-normal or subnormal levels. The patients do not have clinical evidence of hypothyroidism at that time (1). Considering the other extreme of the spectrum, most athyroid patients have a negative TRH test only if T4 SC are raised to levels beyond the normal range by large doses of T4. This is associated with T3 SC in the upper normal range. These patients do not appear to be clinically hyperthyroid (2). These findings suggest that a non-thyroidal mechanism accounts for the reciprocal changes in T4 SC and T3 SC. The purpose of this clinical study was to clarify the role of these non-thyroidal mechanisms in regulating T3.
KeywordsTral Hypothyroidism Thyrotoxicosis Thyroxine Triiodothyronine
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