Free triiodothyronine/free thyroxine ratio in children with congenital hypothyroidism

Thyroid-stimulating hormone is generally regarded as a standard parameter for the evaluation of thyroid function. However, relying on this hormone alone can be misleading. Therefore, thyroxine/free-thyroxine levels are used in patients with levothyroxine substitution for the adjustment of therapy. Even with normal values for free thyroxine, decreased values for the free-triiodothyronine/free-thyroxine ratio have already been described in adults. In this study, the free-triiodothyronine/free-thyroxine ratio of 25 children with congenital hypothyroidism was compared with 470 healthy children seen for other reasons and then for thyroid dysfunction. Mean free thyroxine in congenital hypothyroidism was just below the upper limit of normal and significantly higher than in control group. Mean values for free triiodothyronine showed no significant difference between the two groups. The mean value for the free triiodothyronine/free-thyroxine ratio in control group was 3.23. Significantly lower ratios were found in the congenital hypothyroidism group with a mean value of 2.5, due to higher values for free thyroxine compared to free triiodothyronine. Furthermore, an increased free triiodothyronine/free-thyroxine ratio was found at higher thyroid-stimulating hormone values due to lower values for free thyroxine. In this study, we demonstrate that the free triiodothyronine/free-thyroxine ratio was significantly lower in children with congenital hypothyroidism compared to the control group. This is most likely due to the higher values for free thyroxine in this group compared to similar values for free triiodothyronine in both groups. Further studies with differentiated thyroid hormone therapy are needed in order to understand the role of peripheral euthyroidism.