Hi Dawn, There are probably some very logical explanations of why hyperT increases in the late winter to the summer. I was unaware that iodine supplementation for cattle was increased during the winter, but this would probably mean higher iodine content in milk. Humans drink milk for vitamin D and vitamin D status declines to a minimum in February. Since it is an oil vitamin, it is stored in the body from the prior fall, but people run very low by Jan-March. Increased milk and dairy consumption to get vitamin D increases the hyper promoting effects of milk consumption Also, in spring and summer, people start eating more fruit.
Fruit is another negative for hypers because it is copper depleting.
It may deplete copper because of the high iron to copper ratio of most fruits. There could easily be other dietary factors that affect the seasonal variation in hyperT symptoms.