The relationship between serum triiodothyronine and thyroxine concentrations in hyperthyroidism

In toxic nodular goitre relapses of hyperthyroidism after medical therapy probably are more common than in toxic diffuse goitre. It has also been reported that in patients with toxic diffuse goitre a high ratio of triiodothyronine (T3) and thyroxine (T4), initially or during medical treatment, predicts a relapse of the hyperthyroidism after cessation of therapy. We therefore studied the relationship between T3 and T4 in untreated patients with toxic diffuse goitres (n = 46, mean ratio T3/T4 29.6 nmol/mumol +/- 10.7 SD) and toxic nodular goitres (n = 12, ratio 29.3 +/- 17.1), and found no significant difference. Both groups differ significantly from normal controls (n = 16, ratio 14.6 +/- 1.5, P < 0.01). From the patients with toxic diffuse goitres we compared two groups. Patients in the first group remained in remission after short-term medical treatment (n = 10); the second group contains patients with a relapse of hyperthyroidism (n = 10). Differences between both groups in the median ratio of T3 and T4 were assessed before the start of treatment, at 4 and at 8 weeks. No significant differences were found between the two groups. The ratio of T3 and T4 is not helpful in distinguishing diffuse and multinodular toxic goitre or in determining the prognosis after medical treatment of hyperthyroidism caused by a hyperfunctioning thyroid gland. However, recurrence of hyperthyroidism was found in 3 patients with a T3/T4 ratio > 60 nmol/mumol after 8 weeks of treatment.