THYROXINE WITHDRAWAL IS ACCOMPANIED BY DECREASED CIRCULATING LEVELS OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-1 IN THYROIDECTOMIZED PATIENTS

We carried out a study on the effect of T4 withdrawal on serum insulin -like growth factor-binding protein-1 (IGFBP-1) levels in 10 thyroidec tomized patients with papillary thyroid cancer receiving T4 replacemen t therapy. As controls we also measured serum sex hormone-binding glob ulin (SHBG) levels, known to be regulated by T4. Each patient acted as his/her own control, studied both before and 30 days after T4 withdra wal. On the average, the IGFBP-1 level decreased by 36% from 66 +/- 9 to 44 +/- 8 mug/L (mean +/- SE; P = 0.0007) during T4 withdrawal. The SHBG level decreased by 47% from 78 +/- 17 to 46 +/-13 nmol/L (P = 0.0 001). The fasting insulin levels were unaffected by T4 withdrawal. The re also was an 18% decline in the serum IGF-I concentration after T4 w ithdrawal (P = 0.011). It is concluded that in athyreotic patients rec eiving T4, withdrawal of the drug decreases serum IGFBP-1 and SHBG con centrations and IGF-I levels. In view of the possible role of IGFBP-1 in glucose counterregulation, these results indicate a novel mechanism by which T4 may exert its metabolic action on liver carbohydrate meta bolism.