Why can't someone with hypothyroidism take melatonin ?

Who says you can't?: Hypothyroidism is a very common disease and very very easily treated. If you are not treated, you probably having some fatigue/tiredness and so taking melatonin can make you are tired, so be careful--but it is not a contraindication. If you are treated and "in normal range" of thyroid (euthyroid) now, melatonin should be ok. So, consult doc. Good luck.