Serum thyroglobulin concentrations in the first weeks of life as measured with an immunoluminometric assay

In addition to the determination of thyrotropin (TSH) on the 5th day of life as a screening parameter for congenital hypothroidism, serum thyroglobulin concentrations were first measured with a commercially available immunoradiometric assay in those cases presenting with elevated thyrotropin levels. As this thyroglobulin assay required at least 400 microliter serum for a duplicate determination, it was decided to employ a sensitive immunoluminometric assay (detection limit 50 amol/tube) instead, the amount of serum needed being reduced to 100 microliter, the sensitivity to under 3 micrograms/l. We describe the serum thyroglobulin concentrations determined by the immunoluminometric assay in both cord and venous blood and in both full-term and pre-term babies divided into 4 main groups with respect to thyroid function. The criteria for the groups with thyroid dysfunction were determined as a result of a) isolated thyrotropin elevation measured in the dry blood spot test (thyrotropin above 4 mU/l), b) lowered thyroxine:thyroxine binding globulin ratio together with elevated thyrotropin both prior to and under substitution with L-thyroxine. In full-term babies thyroglobulin levels in serum fell steadily over the first months of life. The same effect was seen in pre-term babies. The effect of L-thyroxine on the suppression of serum thyroglobulin levels appeared to be dose-dependent, as newborns from experimental criterium group a) (defined above) showed no suppression of serum thyroglobulin levels when under partial substitution with L-thyroxine (median 84 micrograms/l), whereas those in group b), i.e. with full L-thyroxine substitution showed a noticeable suppression of serum thyroglobulin (median 27 micrograms/l).