This Canadian paper demonstrates the linkage between fluoride ingestion and effects on thyroid function.
Science of the Total Environment:
Fluoride exposure and hypothyroidism in a Canadian pregnancy cohort
Meagan Hall , Bruce Lanphear , Jonathan Chevrier , Rick Hornung , Rivka Green , Carly Goodman , Pierre Ayotte , Esperanza Angeles Martinez-Mier , R. Thomas Zoeller , Christine Tillmore