The thyroidal T4/T3 ratio and its regulation in non-toxic goitre

In 35 patients with non-toxic goitre, surgically resected thyroid tissue was hydrolyzed with Pronase under anaerobic conditions in the presence of methylmercaptoimidazole. Total iodine, PBI, L-thyroxine (T4-RIA and T4 (D)) as well as triiodothyronine (T3-RIA) were determined in the tissue hydrolysates. The data obtained were compared with T4/T3 ratios in the serum, TRH test and thyroidal 131I uptake before operation. The total iodine, amounted to 112.6 +/- 14.6 ug/g tissue. The T4/T3 ratio was 11.37 +/- 1.80. Using this procedure of tissue hydrolysis and RIA assays for iodothyronines, the yield for T4 and T3 is considerably higher than the one obtained with previous methods. As in animal experiments, evidence was given for human goitre that T3 is predominantly produced first in the case of decreased T4 production. Second in the case of iodine deficiency, and third in cases of increased TSH response after TRH. Patients with a large amount of iodine and T4 in their thyroids have a much smaller increase in serum TSH after TRH than do subjects with a much lower thyroidal T4 concentration. This occurs even though the thyroid hormone levels in serum in these groups are not different. No correlation between thyroidal T4/T3 ratio and serum T4/T3 ration could be demonstrated even under extreme conditions. This finding favors the assumption of a predominant extrathyroidal regulation for T4/T3 ratio in serum.