BONE-MINERAL DENSITY AND METABOLISM IN CHILDREN WITH CONGENITAL HYPOTHYROIDISM AFTER PROLONGED L-THYROXINE THERAPY

The effect of long-term L-thyroxine (LT4) replacement therapy on bone mineral density and on biochemical markers of bone turnover were studi ed in children with congenital hypothyroidism (CH). Forty-four childre n and adolescents (mean age 8.5 +/- 3.5 years) with primary CH who beg an LT4 replacement therapy within the first month of life were studied . Bone mineral density (BMD) of the lumbar vertebrae and the upper fem oral bone was measured by dual energy X-ray absorptiometry. Serum oste ocalcin (OC) and bone alkaline phosphatase were measured as markers of bone formation and urinary deoxypyridinoline was taken as a marker of bone resorption. Bone mineral densities of CH children were not diffe rent from those in age-matched controls. The biochemical markers of bo ne turnover were normal except for the serum OC levels which were foun d to be higher than in controls and positively correlated with the fre e thyroid hormone levels (for FT4 r = 0.42, p = 0.02). Eight CH childr en demonstrated low BMD values (below -1 SDS) at -2 +/- 0.7 SDS for th e lumbar spine and - 1.6 +/- 0.5 SDS for the femoral site. These eight children showed lower mean weight (p < 0.05) and their dietary calciu m intake tended to be less (p < 0.06) than that seen in the normal BMD group. In conclusion, our results show that LT4 replacement therapy f or 8 years is not detrimental to the skeletal mineralization of CH chi ldren. As in a healthy population, weight and current intake of calciu m seem to be major determinants of bone density. Dietary recommendatio ns, especially when calcium intake is below the recommended dietary al lowance, may have to be reconsidered.