Characterization of sex hormone-binding globulin isoforms in hypothyroid women

Liver sex hormone-binding globulin (SHBG) biosynthesis is regulated by triiodothyronine (T3). This regulation is responsible for increased serum SHBG concentrations in hyperthyroid states. However, in hypothyroidism, normal SHBG levels are frequently found. To understand this we have characterized circulating SHBG isoforms according to their sialic acid content, which determines its half-life, in euthyroid and hypothyroid women. Six euthyroid (aged 56 +/- 8 years) and five hypothyroid women (51 +/- 13 years) were studied. Their body mass index (BMI) range was 20-25. Hypothyroidism diagnosis was based on clinical findings, elevated basal thyrotropin (TSH) and decreased T3 and thyroxine (T4) values. Total SHBG was measured by radioimmunoassay (RIA) and SHBG isoforms were isolated using preparative isoelectrofocusing. For comparisons, two-tailed t test was applied. No statistical difference was found between the total SHBG levels of hypothyroid and euthyroid postmenopausal women. Three groups of SHBG isoforms were isolated in the euthyroid group: S(I): pl: 5.0-5.2: 20% +/- 4%, S(II) : pl 5.2-5.4: 50% +/- 3% and S(III): pl 5.4-5.6: 29% +/- 4%. In hypothyroid patients, although the three groups of isoforms were isolated in the same pH range, S(I) and S(II) proportions were different (p < 0.001) when compared to normal women: S(I): 34% +/- 4%, S(II): 33% +/- 9.9% and S(III): 29% +/- 5.7%. These results show that hypothyroid patients have a higher proportion of more acidic SHBG isoforms. This variation may explain the normal levels of serum SHBG observed in hypothyroidism.