[Clinical studeis on the regulatory system of thyroid hormone secretion and serum triiodothyronine. Part III. Studies on thyroid hormone change and its relationship when TSH in cases with subclinical hypothyroidism (Hashimoto's thyroiditis and 131I treated Graves' disease)].

Changes in thyroid hormone levels and their relationship with TSH in cases with Hashimoto's thyroiditis and 131I treated Graves' disease in subclinical hypothyroid state were investigated: 1) In 34 cases with Hashimoto's thyroiditis, TSH levels was 14.7 +/- 26.8 muU/ml (mean +/- s.d.), T3 144.5 +/- 33.6 Rg/100 ml; T4 6.7 +/- 2.4 MUG/100 ML; T3RU 25.4 +/- 3.2%; F.T.I. 2.0g +/- O.89; T3/T4 2.46+/- 1.04%. It was noted that TSH levels revealed significant positive correlation with T3/T4 (r=0.77); significant inverse correlation with F.T.I. (r= - 0.51); and non-significant inverse correlation with T3 levels (r= - 0.06). These results indicate that T3 might play a more important role than T4 in maintaining normal thyroid function (so-called T3 euthyroidism). This is probably based on metabolic disturbance of iodine which is common in Hashimoto's thyroid. In several cases with rather high normal basal T3 levels, no significant elevation in T3 was observed in spite of elevated TSH level following TRH administration. This might indicate that the thyroid is fully functioning and that no more reserve is left for the thyroid to respond further to elevated TSH. 2) In 30 cases with 131I treated Graves' disease, TSH concentration was 22.3 +/- 29.8 muU/ml (mean +/- s.d.); T3 100.7 +/- 35.4 ng/100 ml: T4 6.2 +/- 2.9 MUG/100 ML; T3RU 25.6 +/- 2.3%; F.T.I. 1.87 +/- 0.85; T3/T4 1.80 +/- 0.64. Significant inverse correlation was noted between TSH levels and F.T.I. as well as T3 levels. Although TSH levels showed significant positive correlation with T3/T4 (r=0.44), its degree of correlation was not as good as that with F.T.I. (r= - 0.71) and T3 (r= - 0.53). These results indicate that there is no preference in T3 production. From the responsiveness of thyroid and pituitary following TRH administration, assumption can be drawn that 131I treated Graves' patients, who became euthyroid, may in fact be on the way to hypothyroidism.