The Thyroidal T₄/T₃ Ratio and its Regulation in Non-toxic Goitre

Abstract

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 ratio could be demonstrated even under extreme conditions. This finding favors the assumption of a predominant extrathyroidal regulation for T₄/T₃ ratio in serum. FT4I for free L-thyroxine index; MMI for methylmercaptoimidazole; PBI for protein bound iodine; TI for total iodine; T3 for L-triiodothyronine; T4 for L-thyroxine; TRH for thyrotropin releasing hormone; TSH for thyrotropin.

Key words

Non-toxic Goitre - lntrathyroidal Iodothyronines - T₄/T₃ Ratio - Iodine Deficiency