Radio-immuno- and paper-electrophoretic analysis of 3,5,3'-L-tri-iodothyronine-binding, proteins in normal human sera

Abnormal gamma globulin binding of thyroid hormones

Thyroid hormone levels were studied in a thyrotoxic patient, who was treated with propylthiouracil. He had heavily increased triiodothyronine concentrations, measured by radioimmunoassay, in spite of only mild clinical symptoms of thyrotoxicosis. A moderately increased serum triiodothyronine concentration was observed in another patient, who was euthyroid and who had recently recovered from subacute thyroiditis. By gel electrophoresis and precipitation tests with human anti-IgG and anti-IgA, a binding to the gamma globulins of both triiodothyronine and thyroxine was detected in patient 1, and of triiodothyronine in patient 2. Such abnormal binding may result in serious errors in the determination of thyroid hormone concentration by radioimmunoassay.

Serum concentration of unsaturated thyroxine-binding globulin in hyper- and hypothyroidism

The concentration of serum thyroxine (T4)-binding globulin (TBG) not binding T4 (unsaturated TBG, u-TBG) was determined in hyper- and hypothyroidism. u-TBG was expressed as the product of TBG concentration and the ratio of free TBG capacity to maximal TBG capacity as determined by reverse-flow electrophoresis. u-TBG concentration in normal sera (n = 40) was 15.5 +/- 2.3 mg/l (mean +/- SD), or 257 +/- 38 nmol/l for a molecular weight of TBG of 60 000 daltons. u-TBG levels were significantly lower in hyperthyroidism (7.1 +/- 2.3 mg/l, n = 16, P less than 0.001) and higher in hypothyroidism (21.7 +/- 5.0 mg/l, n = 22, P less than 0.001). Based on partial correlation analysis, u-TBG was inversely correlated to serum T4 (r = -0.586, P less than 0.001), but not correlated to triiodothyronine (T3) (r=-0.180, NS). There was a reciprocal correlation between u-TBG concentration and the T3 uptake value (r = 0.748, P less than 0.001). There was also a reciprocal correlation of u-TBG with both % free T4 (r = 0.425, P less than 0.001) and % free T3 (r = 0.377, P less than 0.001), when the data were subjected to partial correlation analysis. These results provide the values for u-TBG concentration in hyper- and hypothyroidism, and support the concept that the free fractions of serum thyroid hormones may be determined by the number of binding sites of the TBG molecule that are not saturated with T4 in hyper-and hypothyroidism.

Thyroid hormone binding by human serum prealbumin (TBPA). Electrophoretic studies of triiodothyronine-TBPA interaction

Thyroxine-binding prealbumin (TBPA) in normal human serum has been shown in a polyacrylamide gel electrophoresis system to bind 7-9% of tracer level purified [(125)I]triiodothyronine (T3), and more than 30% of T3 in serum deficient in thyroxinebinding globulin (TBG). The T3-TBPA interaction has been confirmed at pH 9.0 and pH 7.4 in this electrophoretic demonstration of TBPA binding of T3 in serum. Purified human TBPA has also been shown to bind T3. Progressive additions of unlabeled thyroxine (T4) to serum containing tracer [(125)I]T3 displace T3 from TBG, its principal carrier, to TBPA and albumin; however, T4 loading does not lead to significant T3 displacement from TBPA even at T4 levels known to saturate TBPA. Loading of serum with unlabeled T3 results in displacement of more than 50% of [(125)I]T3 from TBPA, as well as from TBG, to albumin. Studies carried out with serum containing diphenylhydantoin (DPH) or MK-185, known inhibitors of T4 binding by TBG, also showed T3 displacement from TBG to TBPA and albumin. Although salicylate and tetraiodothyroacetic acid (TETRAC) displace T4 from sites on TBPA, they have only minimal effects on T3-TBPA interaction.