I believe the mixed Iodine results from the later decades are influenced by the increased amount of pufas;
" As pointed out by Stephen Guyenet in his Whole Health Source blog, omega-6 fatty acids may suppress thyroid signaling. He cites studies showing that corn oil greatly suppresses the liver’s response to T4 when compared to lard, safflower oil suppresses the liver’s response to T3 when compared to beef tallow, and linoleic acid suppresses the response of brown fat and the liver to T3. The liver is one of the main sites of thyroid hormone-responsive heat production. In fact, in the 1970s researchers were considering omega-6 lineleic acid as a treatment for hyperthyroidism."