MARKERS OF BONE TURNOVER IN PATIENTS WITH DIFFERENTIATED THYROID-CANCER WITH AND FOLLOWING WITHDRAWAL OF THYROXINE SUPPRESSIVE THERAPY

Objective: To study whether levothyroxine (LT4) suppressive therapy ex poses patients with differentiated thyroid cancer (TC) to an increased risk of osteoporosis. Design and Methods: Markers of bone formation ( serum alkaline phosphatase (ALP), osteocalcin (OC), type I procollagen carboxyterminal (PICP) and aminoterminal (PINP) propeptide) and resor ption (serum type I collagen carboxyterminal telopeptide (ICTP) and ur ine hydroxyproline (HOP)), as well as serum intact parathyroid hormone (PTH), 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D (1,25(OH)(2)- D) were measured in 29 patients (25 women, 4 men) with a median age of 45 years, and in 38 age- and sex-matched controls. In a subgroup of 1 4 patients the measurements were repeated after 5 weeks' interruption of LT4 therapy. Since the primary treatment of TC the patients had use d TSH suppressive doses of LT4 (a mean daily dose of 215 mu g) for 9 t o 11 years. The bone mineral density (BMD) of patients and controls wa s measured by dual energy X-ray absorptiometry. Results: When on T-4 t herapy patients had significantly higher mean levels of ALP (+21%, P < 0.05), OC (+35%, P < 0.01), PICP (+10%, P < 0.05), PINP (+46%, P < 0, 001), ICTP (+21%, P < 0.05), and HOP (+37%, P < 0.001) compared with c ontrols. After stopping treatment, OC (-42%, P < 0.001), PINP (-7%, P < 0.05), and ICTP (-54%,P < 0.001) decreased, whereas PICP (+24%, P < 0.001) and 1,25(OH)(2)D (+29%, P < 0.01) increased. BMD of the lumbar spine and the upper femur was similar in patients and controls. Conclu sions: Patients with differentiated TC have high bone turnover when on LT4 suppressive therapy. After withdrawing treatment both bone format ion and resorption decrease acutely. During development of hypothyroid ism, serum PICP and PINP, which form from the same type I procollagen molecule and should change similarly, behaved differently. This may be due to different effects of hypothyroidism on their removal through s eparate receptors in the liver.