Measurement of serum thyroxine binding prealbumin in various thyroidal states by radioimmunoassay.

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Abstract
Serum thyroxine binding prealbumin (TBPA) levels in various thyroidal states were examined by radioimmunoassay (RIA). This technique is highly sensitive, accurate and reproducible. The normal mean (.+-. 2 SD) level of serum TBPA is 26.9 .+-. 8.0 mg/dl (29.4 .+-. 5.2 in men and 24.9 .+-. 7.6 mg/dl in women). Serum TBPA levels in pregnant women were significantly lower than in normal females (P < 0.05). Serum TBPA levels in patients with untreated hyperthyroidism were 12.9 .+-. 4.0 mg/dl (mean .+-. SD) and in patients with untreated hypothyroidism were 25.2 .+-. 4.7 mg/dl (mean .+-. SD). The mean TBPA concentrations in untreated hyperthyroidism were significantly lower than that for normal population (P < 0.01), but untreated hypothyroidism was almost within normal range. The changes in TBPA levels in hyperthyroidism and hypothyroidism were similar to those in TBG [thyroid binding globulin] levels. In untreated hyper- and hypothyroidism, restoration to euthyroidism by treatment was uniformly accompanied by a normalization of serum TBPA and TBG levels. A negative correlation between serum thyroid hormone binding protein (TBG and TBPA) and free thyroxine was observed in patients with hyperthyroidism. The coefficient of correlation between TBPA and free thyroxine was -0.80 (P < 0.01) and between TBG and free thyroxine -0.58 (P < 0.01). It appears that not only TBG but also TBPA may play an important role in the regulation of the free thyroxine concentration in response to various thyroidal states.