In this study we measured serum osteocalcin (Bone Gla-Protein) to investigate bone metabolism in pregnant women with multinodular thyroid goiter undergoing treatment with levothyroxine (L-T4). Serum concentration of BGP was measured in 18 pregnant women and in 20 non-pregnant women (ages raging from 21-34 years) receiving L-T4 (75-125 micrograms/day). Venous blood samples for RIA determination of serum BGP, plasma thyroid hormone (T3, T4, free T3, free T4) and TSH were collected from the two groups. The samples of the pregnant women group were collected before pregnancy (at the moment the disease was diagnosed without L-T4 therapy) during pregnancy (at the 3rd, 6th and 9th month) and one month after delivery. The normal TSH levels (measured with Irma method) before pregnancy, were significantly reduced during treatment with L-T4 during pregnancy and after delivery (p < 0.005), respectively. Also in the control group TSH levels were reduced during treatment. Serial measurement of serum BGP before pregnancy, (3.4 +/- 1 ng/ml) during pregnancy (3rd: 4.2 +/- 1.5 ng/ml; 6th: 4.2 +/- 1.4 ng/ml; 9th: 2.8 +/- 1.6 ng ml, month respectively) and one month after delivery (3.5 +/- 1.3 ng/ml) did not demonstrate significant variations. Furthermore, in the control group the BGP levels were 3.2 +/- 1.7 ng/ml. There was no correlation between BGP, thyroid hormones and TSH in these groups. These data indicate that the administration of moderate doses of L-T4 in pregnant and in non-pregnant women did not modify the serum BGP levels.