Most folks bodies have been deficient in Iodine for so long, that when you suddenly give it some, the cells and systems involved in its uptake become suddenly awakened. TSH skyrocketing suddenly in response to suddenly giving the body what it needs that it hasn't had for so long, elicits measurable responses and one of them is a sudden increase in TSH, which is well-documented, and which is well known that the vast majority of "professional" endocrinologists mis-interpret as alleged hypothyroidism.