In areas of high iodine intake, increased fluoride levels are associated with a lowering of serum iodine levels, and higher iodine excretion rates.
(The basis for the fluoride treatment in cases of iodine-induced hyperthyroidism). This can be seen in urinary iodine (UIC) levels.
Serum fluoride levels correlate with UIC levels.
Here is another graph by Shashi and Singla (2013).
Singla S, Shashi A - "Thyroid peroxidase activity as toxicity target for fluoride in patients with thyroid dysfunction" Current Research in Microbiology and Biotechnology 1(2):53-57 (2013)