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 thyroidectomized patients with papillary thyroid cancer receiving T4 replacement therapy. As controls we also measured serum sex hormone-binding globulin (SHBG) levels, known to be regulated by T4. Each patient acted as his/her own control, studied both before and 30 days after T4 withdrawal. On the average, the IGFBP-1 level decreased by 36% from 66 +/- 9 to 44 +/- 8 micrograms/L (mean +/- SE; P = 0.0007) during T4 withdrawal. The SHBG level decreased by 47% from 78 +/- 17 to 46 +/- 13 nmol/L (P = 0.0001). The fasting insulin levels were unaffected by T4 withdrawal. There also was an 18% decline in the serum IGF-I concentration after T4 withdrawal (P = 0.011). It is concluded that in athyreotic patients receiving T4, withdrawal of the drug decreases serum IGFBP-1 and SHBG concentrations 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 metabolism.