Summary, In order to establish a direct method for the determination of thyroxine-binding globulin, the preparation of TBG was performed by five sub sequential chromatographical steps: first, TBG was concentrated 640-fold from pooled human serum by affinity chromatography on agarose to which triiodothyronine was covalently bound over six carbon long spacer groups using the bromine cyanide activation method. The further purifieation was performed by anion exchange chromatography on QAE-and DEAE-Sephadex A-50, and affinity chromatography for glycoproteins on ConA-Sepharose. The final enriehment of TBG was 4500 fold, the overall yield was approximately 20%. This preparation was shown to be highly purified by disc eleetrophoresis (three different coneentrations of polyaerylamide and two different pH values) and by equimolar bin ding for T 4 and T 3' Microheterogeneity was shown by three different protein bands in isoeleetric foeusing in the pH range from 4.0 to 4.5. The indirectly calculated content of carbohydrates was about 15%.This TBG preparation was used for immunization of rabbits and for labelling with 125iodide by the chloramine T-method. The inter assay precision of the radioimmunoassay was satisfactory, the eoefficient of variation being 4.3%. Serum dilution curves were found to be on the standard curve, proving no immunological heterogeneity, also in sampies from subjeets with quantitative TBG-variations.The normal range of TBG levels (23.0 ± 4.0 mg/I, x ± SD) was shown to be in good agreement with Mit Unterstützung des Sonderforschungsbereichs 51 Medizinische Molekularbiologie und Biochemie ** Diese Arbeit wurde mit dem ersten "von Basedow-Forschungspreis Schilddrüse" 1977 der Deutschen Gesellschaft für Endokrinologie ausgezeichnet the literature. But there existed a significant age dependency: The high TBG levels in the neonatal period were found to deerease until the age of 15 years remaining at this low level up to 50 years. Thereafter they increased continously. After the neonatal period there was a significant correlation between total T 4 and TBG levels in serum. Consequently the T 4/TBG ratio remained eonstant (3.2±0.7, x±SD), eliminating the age dependency of both, T 4 and TBG for diagnostic purposes.There existed no sex differenees in any period of life and no fluetuations during menstrual cycle. The estrogen indueed rise of TBG levels could be confirmed quantitatively in females (pregnaney and estrogen therapy), similarly in males TBG inerease could be demonstrated during fosfestrol therapy. In drug indueed states of estrogen deficieney inversely a decrease of TBG levels could be shown in females during danazol therapy. These estrogen influences on TBG seem to be long term effects.Variations ofTBG levels in primary thyroid disorders reported in the literature eould not be eonfirmed by the direct radioimmunological measurement: In hyperthyroidism (20.0 ± 3.5 mg/I), endemie goiter (21.1 ± 4.6 mg/I) and hypothyroidism (21.6 ± 7.0 mg/I) TBG was essentially in the normal range. In these states of th...