Williams 2017.

Methods	Randomised controlled trial Study sites: 21 neonatal units in the UK (2010–2012)
Participants	Newborn infants < 31 weeks' gestation at birth
Interventions	Treatment (n = 637): sodium iodide given enterally or parenterally at 30 μg/kg/day Control (n = 636): sodium chloride given enterally or parenterally at 30 μg/kg/day Allocated trial solution continued until infants reached 34 weeks' gestational age
Outcomes	Primary: neurodevelopmental status defined by the Cognitive, Language, and Motor domains of the Bayley‐III at 2 years of age corrected for prematurity Secondary: *mortality, subtests of Bayley‐III; Bayley‐III analysed as a dichotomous outcome (death or Bayley‐III score < 85 in any of the main domains versus Bayley‐III score ≥ 85); plasma levels of T4, TSH, TBG up to 34–36 weeks' postmenstrual age; neonatal morbidity (RDS, CLD, PDA, NEC, hyperbilirubinaemia, acquired infections, cerebral pathology closest to 34 weeks', porencephalic cyst, cystic periventricular leukomalacia, ventriculopleural)
Notes	1275 infants randomised. 2 infants randomised in error (n = 1273), 14 withdrawals (6 in iodine group, 8 in placebo group, no data used); sample size of 631 in iodine group and 628 in placebo group (n = 1259) *(61 infants in iodide group and 59 infants in placebo group died during the intervention stage of the trial, and 65 infants in iodide group and 66 infants in placebo group died before 2‐year follow‐up).
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation programme using "bespoke minimization algorithm to ensure balance across hospitals on gender and gestational age."
Allocation concealment (selection bias)	Low risk	Allocation via "a secure website with telephone backup."
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Quote: "Packaging and visual appearance of trial solutions were identical." Research team, trial statistician, parents, neonatal staff, and pharmacy blinded to content of trial solutions.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Outcomes assessment blinded to trial group allocation.
Incomplete outcome data (attrition bias) All outcomes	Low risk	< 10% loss to follow‐up.
Selective reporting (reporting bias)	Unclear risk	Days of endotracheal intubation and days of continuous positive airways pressure prespecified but not reported.

Bayley‐III: Bayley Scales of Infant and Toddler Development, Third Edition; CLD: chronic lung disease; n: number of participants; NEC: necrotising enterocolitis; PDA: persistent ductus arteriosus; RDS: respiratory distress syndrome; T4: free thyroxine; TBG: thyroid‐binding globulin; TSH: thyroid‐stimulating hormone.