Thyroid hormone metabolism in thyroid disease as reflected by the ratio of serum triiodothyronine to thyroxine

Abstract

In 77 euthyroid subjects the mean ratio between serum triiodothyronine (S-T₃) in nmol/l × 10³ and serum thyroxine (S-T₄) in nmol/l was 22.4 with a standard deviation of 4.3. The distribution of the ratio was close to Gaussian. It was not changed in subjects under estrogen effect. The ratio was increased in hypothyroid subjects suggesting a mechanism preserving S-T₃ better than S-T₄ in thyroid failure. This mechanism seemed independent of TSH as the increasing S-T₃/S-T₄ ratio in primary hypothyroidism was not correlated to serum TSH (S-TSH), and patients with hypothyroidism of pituitary origin, without any increase in S-TSH, had a significantly elevated S-T₃/S-T₄ ratio. In subjects taking thyroxine, S-T₄ but not S-T₃ increased with the dose of thyroxine, leading to decreasing S-T₃/S-T₄ ratios. This indicates a mechanism that keeps S-T₃ within normal limits also when the S-T₄ supply is high. This applied also to athyreotic subjects on thyroxine and to subjects on thyroxine plus carbimazole, suggesting that the mechanism is independent of thyroid tissue, uninfluenced by carbimazole, and probably operates via the peripheral metabolism of T₄. Changes in the S-T₃/S-T₄ ratio can be achieved by several mechanisms, and the specificity of ratio changes is low. However, by investigating the ratio S-T₃/S-T₄ in selected clinical states we can depict the ability of the organism to protect its normal S-T₃ level in situations with divergent S-T₄ concentrations, even if details about the mechanisms cannot be explained.