In vitro, thyroxine was found to increase collagen I expression in fibroblasts of patients with KC but not in keratinocytes, while it had no effect on the production of transforming growth factor β1 (TGF-β1) or collagen V in either cell population, thus making it difficult to think that increased thyroxine concentration alone could play a role in the development of KC [200]. Here, TGFB1 is linked to keratoconus.