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Selenium Decreases Thyroglobulin Concentrations But Does Not Affect the Increased Thyroxine-to-Triiodothyronine Ratio in Children with Congenital Hypothyroidism¹

Children’s Hospital Reine Fabiola (J.-P.C.), Laboratory of Pediatrics (P.B.), B 1020 Brussels, Belgium; Institute of Pharmacy (J.N.), B 1090 Brussels, Belgium; Centre Inter Universitaire Hôpital Ambroise Paré, Mons (J.V.), Free University of Brussels, B 7000 Brussels, Belgium; and Nuclear Medicine Medical Service (S.Y.W.), Veterans Affairs Medical Center, Long Beach, California 90822

Address all correspondence and requests for reprints to: Jean-Pierre Chanoine, M.D., Endocrinology and Diabetes Unit, British Columbia’s Children’s Hospital, 4480 Oak Street, Room 1A46, Vancouver V6H 3V4, Canada. E-mail: jchanoine{at}cw.bc.ca

Compared with euthyroid controls, patients with congenital hypothyroidism (CH) who are receiving L-T₄ treatment show elevated serum TSH relative to serum T₄ concentrations and increased T₄/T₃ ratio. These abnormalities could be the consequence of impaired activity of the selenoenzymes deiodinases on which patients with CH rely to convert the ingested L-T₄ into active T₃. Eighteen patients (0.5–15.4 yr), diagnosed with CH in infancy, received selenomethionine (SeM, 20–60 µg selenium/day) for 3 months. The study took place in Belgium, a country where selenium intake is borderline. Compared with the values observed in age- and sex-matched euthyroid controls, patients with CH had decreased selenium, thyroglobulin and T₃ concentrations and increased TSH, reverse T₃, and T₄ concentrations and T₄/T₃ ratio at baseline. Selenium supplementation caused a 74% increase in plasma selenium values but did not affect the activity of the selenoenzyme glutathione peroxidase used as a marker of selenium status. SeM abolished the TSH difference observed between CH patients and euthyroid controls at baseline and caused a significant decrease in thyroglobulin values. Thyroid hormone concentrations were not affected by SeM. In conclusion, our data suggest that selenium is not a limiting factor for peripheral T₄-to-T₃ conversion in CH patients. In contrast, we find indirect evidence that SeM improves thyroid hormones feedback at the hypothalamo-pituitary level and decreases stimulation of the residual thyroid tissue, possibly suggesting greater intracellular T₄-to-T₃ conversion.

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