The behavior of the triiodothyronine/thyroxine (T3/T4) ratio in normal individuals, and its implications for the regulation of euthyroidism.

In order to examine the significance of differences in the triiodothyronine/thyroxine (T3/T4) ratio in the control of euthyroidism in a normal population, we measured the serum T3, T4 and FT4 index (FT4I) in 121 healthy men and 68 women. The correlation between T3 and T4 was poor (r = 0.24), suggesting that most of the natural variation in T3 among normal individuals occurs independently of the T4 level. There was, however, a strong inverse correlation (r = 0.59, p less than 0.001) between T3/T4 ratio and FT4I. This correlation reflected a constancy in mean T3 at different levels of T4 such that the T3 changed by only 0.35 nmol/l (22%) throughout the range of normal T4. Furthermore, the T3/T4 ratios of 48 hypothyroid individuals with subnormal T4 represented a simple extension of the relationship found in the healthy individuals, indicating that the rise in T3/T4 classically associated with falling T4 in primary hypothyroidism is merely part of a continuous process of change which begins in euthyroidism. It is proposed that the relationship between T3 and T4 is finely controlled in healthy individuals in such a way that differences in hormone activity due to the wide range of serum T4 among normals may be buffered by systematic adjustment of the T3/T4 ratio. Those with the highest T4 appear to maintain euthyroidism with a minimal T3/T4 ratio. The remainder, with T4 levels towards the lower limit of normal, are subject to a progressive increase in the T3 fraction and may depend on it to remain euthyroid.(ABSTRACT TRUNCATED AT 250 WORDS)