Endocrine Abstracts

Thyroid hormone (TH) synthesis requires iodine, a scarce element whose recycling is mediated by DEHAL1 through deiodination of iodotyrosines MIT and DIT. In humans, DEHAL1 defects lead to severe congenital hypothyroidism (CH) non-detected by neonatal screening programs, which involves the risk of mental retardation in infants. The timing for establishment of this hypothyroidism remains unknown, but environmental iodine deficiency may represent a triggering factor. While urinary loss of iodotyrosines could represent early biomarkers for the disorder, its determination in fluids remains a technical challenge.

Objective: To measure urinary concentrations of iodotyrosines and test their correlation with urinary iodide in Dehal1 knockout mice under experimentally controlled iodine deficiency.

Methods: Dehal ^(−/−) and wilttype (wt) mice were subjected to normal, low, and very low iodine diets (NID, LID, VLID) containing 5.6, 1 and 0.25 μgI/day, for 28 days. At d0, d15 and d28, urinary iodine concentration (UIC) was determined by Kolthoff method and MIT, DIT determined in urine using a novel LC/MS-MS protocol.

Results: At baseline andNID, MIT and DIT urinary levels were statistically higher in Dehal1^(−/−) (5 and 8 ng/ml) than in wt (1 and 0.5 ng/ml), respectively (P<0.02). Accordingly, UIC was increased in Dehal1^(−/−) (30 mg/dl) compared to wt (15 mg/dl) (P<0.05), suggesting that iodide content of iodotyrosines is freed during Kolthoff procedure and adds to the general iodide pool in the urine. This situation persists in time till d28. Under LID, MIT and DIT concentrations remain elevated in Dehal1^(−/−) (5.5 and 8 ng/ml) compared to wt (0.5 and 0.5 ng/ml) (P<0.05) at d15, but start to decrease in both genotypes at d28. UIC was still higher in Dehal1^(−/−) (4 mg/dl) versus wt (1 mg/dl) (P<0.05) at d15 and d28, but decreased with respect to NID. Under VLID, MIT and DIT levels were still remained significantly higher in Dehal1^(−/−) (4 and 3 ng/ml) compared to wt (0.5 and 0.25 ng/ml (P<0.05) at d15. At d28, iodotyrosines lowered to barely detectable levels in wt mice. Finally, UIC levels in Dehal1^(−/−) were intensely decreased (0.5 mg/dl) at d15 and d28, while wt mice showed undetectable levels, reflecting stringent iodine restriction.

Conclusion: Our data strongly suggest that the accurate measurement of increased urinary loss of MIT and DIT correlates with UIC increment in Dehal1 deficient mice, which is triggered by iodine deficient intake. Therefore, urinary iodotyrosines may represent pre-clinical biomarkers for early detection and treatment of DEHAL1 deficiency and prevention of mental retardation risks related to late diagnosis of CH.