Classification of Hypothyroidism in Evaluating Patients after Radioiodine Therapy by Serum Cholesterol, T3‐Uptake. Total T4, FT4‐Index, Total T3, Basal TSH and TRH‐Test*

During a follow‐up examination of patients after radioiodine therapy for thyrotoxicosis, 128 patients without recurrent hyperthyroidism were investigated for the clarification of different degrees of hypothyroidism. The clinical diagnosis and the conventional tests for circulating thyroid hormones were compared to the estimation of serum thyroid stimulating hormone (TSH), and to the thyrotropin‐releasing hormone (TRH) test, which was performed with 500 μg synthetic TRH I.V., and referred to age and sex specific normal ranges. Moreover, serum triiodothyronine (T3) was estimated by radioimmunoassay. Results: 1. A gradual classification of biochemical group‐differences gave more significant discrimination than a division into groups based on clinical impression. Different grades of severity of hypothyroidism could be demonstrated by highly significant differences of free thyroxine index (FT4‐Index) between the 1st group of patients with low FT4‐Index and a 2nd group with raised basal TSH (and normal FT4‐Index), between the 2nd group and a 3rd group with an elevated value of Δ TSHmax (and normal FT4‐Index and basal TSH), and between the 3rd group and a 4th group of biochemically normal reacting patients and controls. There was a less significant difference with very considerable overlap between clinically established groups. 2. T3‐uptake, total T4 and FT4‐Index are not sufficient for detecting hypothyroidism in individual patients, although their group‐differences are significant in biochemical classification. 3. Neither between clinical nor between biochemical groups was there any significant correlation with serum cholesterol. 4. Triiodothyronine can be normal or elevated in a situation with low T4 and raised TSH concentrations. Conclusions: Evidence could be given that hypothyroidism is a graded phenomenon. Its classification by biochemical data is more reasonable than a clinical division. Advancing severity of hypothyroidism after radiation therapy is compensated during a certain period by supplementary production of triiodothyronine. As thyroid hormone concentration in patients with pathological serum TSH or TRH‐test is significantly lower than in euthyroid patients or in controls, replacement therapy in early stages of hypothyroidism also seems reasonable.