Supplementary Material for: Natural History and Management of Congenital Hypothyroidism with in situ Thyroid Gland

Background/Objective: Normally sited glands account for increasing congenital hypothyroidism (CH). Mechanisms often remain unknown. To report the incidence of CH with in situ thyroid gland (ISTG) and describe the natural history of the disease without known etiology. Method: Clinical, biochemical and imaging data at diagnosis were retrospectively analyzed in 285 children positively screened for CH in Ile-de-France between 2005 and 2008. If treatment was discontinued, management of hormonal substitution and follow-up of biochemical thyroid function was performed. Results: 93 full-term CH neonates displayed ISTG (40.6%), including 50 with unexplained mechanism. Follow-up data were available in 32 of them. Therapy was withdrawn from 20 children at a median age of 23.5 months (6-66), among whom 18 remained still untreated over a median duration of 15.3 months (4.4-29.6). In 11 children, levothyroxine (L-T4) dosage was increased over time to maintain biochemical euthyroidism. No statistical differences in initial TSH or FT₄ levels, iodine status or birth weight were found between children with transient and permanent hypothyroidism. Conclusion: Withdrawal of L-T4 substitution was feasible in 56.2% of full-term children with CH with ISTG but unexplained mechanism, emphasizing the need for systematic therapy withdrawal. However, further studies are warranted to standardize withdrawal protocol.