Conversion of thyroxine (T₄) to 3,5,3’-triiodothyronine (T₃) and reverse 3,3’,5’-triiodothyronine (rT₃) was measured in vitro in human placenta and fetal membranes. T₄ (5 µg/ml) was incubated in 0.15 mol/l phosphate buffer with tissue homogenates for 2 h at 37°C, and the T₃ and rT₃ generated were determined in ethanol extract using RIA methods. The placenta and chorion homogenates converted more T₄ to T₃ than to rT₃; the placenta was more active than the chorion. In both tissues the highest converting activity was found in microsomal fractions.

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Thyroidal thyroxine (T4), triiodothyronine (T3), thyroglobulin (Tg) and T4/T3 ratio were investigated in nodular and paranodular tissue from 16 patients with autonomously functioning thyroid adenomas. The concentration of T4 and T3 in the nodule were 97.7 +/- 20.5 (Mean +/- SE) and 10.2 +/- 2.4 micrograms/g wet weight (w.w.). Both iodothyronines were significantly lower in paranodular tissue (22.6 +/- 4.8 and 1.45 +/- 0.32 micrograms/g w.w., respectively), but with disproportionately decreased T3 which resulted in T4/T3 ratio (25.4 +/- 6.4) higher than in adenoma tissue (11.2 +/- 1.6). In patients with high normal or supranormal serum T3 concentration, thyroidal concentration of T3 in adenoma tissue was higher (15.6 +/- 5.0 vs. 6.3 +/- 1.2 micrograms/g w.w.) and T4/T3 ratio lower (8.05 +/- 2.1 vs. 13.2 +/- 1.9) than in patients with normal serum T3. The results suggest that thyroid release of T3 from adenoma is relatively higher than T4 in patients with autonomously functioning thyroid nodules and increased peripheral T3.

In spite of different daily iodine intake in two yugoslav districts (littoral-Split 297 +/- 76 (Mean +/- SD) and continental-Zagreb 111 +/- 36 micrograms/g creatinine, p less than 0.001), thyroidal iodothyronine concentrations and T4/T3 ratio in normal post mortem thyroids were comparable. T4 was 260.0 +/- 42.6 micrograms/g w.w. (Mean +/- SE) in Split group vrs. 279.9 +/- 59.4 in Zagreb. T3 was 12.8-2.0 vrs. 12.8 +/- 2.2 and T4/T3 ratio 21.6 +/- 1.9 vrs. 21.1 +/- 2.9 in Split and Zagreb, respectively. It has been concluded that relatively small difference in iodine intake has no effect on thyroidal concentrations of iodothyronines and T4/T3 ratio.

Daily iodine intake has been investigated in 52 healthy children (5-14 years), 112 healthy adults and in 39 patients with nontoxic goiter from the area of Zagreb/Yugoslavia. Fourteen goitrous patients received 1-thyroxine 150 micrograms daily for at least three months before the examination. Iodine intake has been estimated on the basis of urinary iodine excretion (microgram I-/g creatinine) in the first morning specimen. Iodine excretion in nontreated goitrous patients (92 +/- 30; Mean +/- SD) was significantly lower than in healthy adults (112 +/- 38), while the value in treated goitrous patients (165 +/- 69) was significantly higher than that in nontreated goitrous and healthy adult subjects. The results suggest that Zagreb area, although classified as nonendemic, has borderline iodine intake, and that relative iodine deficiency is of importance in goiter formation. The authors plead for increased daily iodine intake through increased table salt iodisation from actual amount of 10 to 20 to 25 mg KI/kg salt in order to provide an average daily intake of 250 micrograms of iodine.