Selenium decreases thyroglobulin concentrations but does not affect the increased thyroxine-to-triiodothyronine ratio in children with congenital hypothyroidism

Compared with euthyroid controls, patients with congenital hypothyroidism ( CH) who are receiving L-T-4 treatment show elevated serum TSH relative to s erum T-4 concentrations and increased T-4/ T-3 ratio. These abnormalities c ould be the consequence of impaired activity of the selenoenzymes deiodinas es on which patients with CH rely to convert the ingested L-T-4 into active T-3. Eighteen patients (0.5-15.4 yr), diagnosed with CH in infancy, receiv ed selenomethionine (SeM, 20-60 mug selenium/day) for 3 months. The study t ook place in Belgium, a country where selenium intake is borderline. Compar ed with the values observed in age- and sex-matched euthyroid controls, pat ients with CH had decreased selenium, thyroglobulin and T-4 concentrations and increased TSH, reverse T-3, and T-4 concentrations and T-4/T-3 ratio at baseline. Selenium supplementation caused a 74% increase in plasma seleniu m values but did not affect the activity of the selenoenzyme glutathione pe roxidase used as a marker of selenium status. SeM abolished the TSH differe nce observed between CH patients and euthyroid controls at baseline and cau sed a significant decrease in thyroglobulin values. Thyroid hormone concent rations were not affected by SeM. In conclusion, our data suggest that sele nium is not a limiting factor for peripheral T-4-to-T-3 conversion in CH pa tients. In contrast, we find indirect evidence that SeM improves thyroid ho rmones feedback at the hypothalamo-pituitary level and decreases stimulatio n of the residual thyroid tissue, possibly suggesting greater intracellular T-4-to-T-3 conversion.