Coconut Oil and Oxidative Stress
One of the reasons the long chain fatty acids in vegetable oils are so damaging to the thyroid is that they oxidize quickly and become rancid. Food manufacturers know about this propensity towards rancidity and, therefore, highly refine their vegetable oils. Considerable research has shown that trans fatty acids, present when vegetable oils are highly refined (hydrogenated or partially hydrogenated), are especially damaging to cell tissue and can have a negative affect on the thyroid as well as health in general. Because the longer chain fatty acids are deposited in cells more often as rancid and oxidizing fat, impairment of the conversion of thyroid hormone T4 to T3 occurs, which is symptomatic of hypothyroidism. To create the enzymes needed to convert fats to energy, T4 must be converted to T3.
http://www.coconutdiet.com/thyroid_health.htm