Iodotyrosines are biomarkers for preclinical stages of iodine-deficient hypothyroidism in Dehal1 knockout mice.

BACKGROUND: Iodine is required for the synthesis of thyroid hormone (TH), but its natural availability is limited. Dehalogenase1 (Dehal1) recycles iodine from mono- and di-iodotyrosines (MIT, DIT) to sustain TH synthesis when iodine supplies are scarce, but its role in the dynamics of storage and conservation of iodine is unknown.

METHODS: Dehal1 knockout mice (Dehal1KO) were generated by gene-trapping and the timing of expression and distribution was investigated by X-Gal staining and immunofluorescence using recombinant Dehal1-Betagalactosidase protein produced in fetuses and adult mice. Adult Dehal1KO and wild-type (Wt) animals were fed normal and iodine-deficient diets for 1 month and plasma, urine, and tissues were isolated for analyses. TH status was monitored including T4, T3, MIT, DIT, and urinary iodine concentration using a novel LC-MS-MS method and the Sandell-Kolthoff (S-K) technique throughout the experimental period.

RESULTS: Dehal1 is highly expressed in the thyroid, is also present in kidneys, liver, and, unexpectedly, the choroid plexus. In vivo transcription of Dehal1 was induced by iodine deficiency only in the thyroid tissue. Under normal iodine intake, Dehal1KO mice were euthyroid but they showed negative iodine balance due to a continuous loss of iodotyrosines in the urine. Counter-intuitively, the urinary iodine concentration of Dehal1KO mice is two-fold higher than in Wt mice, indicating S-K measures both inorganic and organic iodine. Under iodine restriction, Dehal1KO mice rapidly develop profound hypothyroidism while Wt mice remain euthyroid, suggesting reduced retention of iodine in Dehal1KO mice. Urinary and plasma iodotyrosines were continually elevated throughout their life cycle, including the neonatal period, when pups are still euthyroid.

CONCLUSIONS: Plasma and urine iodotyrosine elevation occurs in Dehal1-deficient mice throughout life. Therefore, measurement of iodotyrosines predicts an eventual iodine shortage and development of hypothyroidism in the pre-clinical phase. The prompt establishment of hypothyroidism upon the start of iodine restriction suggests that Dehal1KO mice have low iodine reserves in their thyroid glands, pointing to defective capacity for iodine storage.