Isn't there numerous studies to the contrary? Starting with this one
""In 1951, Greer reported the pattern of recovery of thyroid function after stopping suppressive treatment with thyroid hormone in euthyroid [normal] subjects based on sequential measurements of their thyroidal uptake of radioiodine. He observed that after withdrawal of exogenous thyroid therapy, thyroid function, in terms of radioiodine uptake, returned to normal in most subjects within two weeks. He further observed that thyroid function returned as rapidly in those subjects whose glands had been depressed by several years of thyroid medication as it did in those whose gland had been depressed for only a few days" (3)
These results have been subsequently verified in several studies. (3)(4) So contrary to what has been stated in the bodybuilding literature, there is no evidence that long term thyroid supplementation will somehow damage your thyroid gland."