Thyroglobulin is a more sensitive indicator of iodine deficiency than thyrotropin: development and evaluation of dry blood spot assays for thyrotropin and thyroglobulin in iodine-deficient geographical areas.

Immunometric assays were developed for thyrotropin and thyroglobulin using time-resolved fluorescence as the measurement signal. The assays were suitable for measurements in serum/plasma or in dry blood spots (3 mm diameter). Both assays have acceptable coefficients of variation for dry blood spots (intra-assay median CV < 10%, interassay CV < 15%) in the concentration range of interest (thyrotropin 3-250 mU/l, thyroglobulin 10-500 micrograms/l). The relatively high CV values are not only due to the assay design but also the inhomogeneity of the samples used. For serum samples the median intra-assay CV was < 3% for thyrotropin in the range 0.1-50 mU/l and for thyroglobulin between 2 and 500 micrograms/l. The corresponding inter-assay CV were less than 5%. The assays were evaluated in field studies carried out under auspices of International Council for Control of Iodine Deficiency Disorders (ICCIDD) with the support of UNICEF in Algeria, Peru, India and Zimbabwe, and were found to be practical inasmuch as dry blood spot samples could be transported without special precautions for up to 5-6 weeks without significant loss in immunoreactivity. This agrees with other findings. The results showed that serum thyroglobulin levels are a more sensitive indicator of iodine deficiency than thyrotropin; elevated thyroglobulin levels were found in 182/304 children in Zimbabwe compared with elevated thyrotropin level in 28/304 cases. 213/304 children had enlarged thyroid glands. The cut-off levels used here were 4.5 mU/l thyrotropin and 20 micrograms/l for thyroglobulin, both in whole blood. The assays proved useful for assessing the efficacy of iodine therapy, either by oral dosage or intramuscularly (iodised oil).(ABSTRACT TRUNCATED AT 250 WORDS)