Our study provides the following key findings: (i) “classic” predictive factors for IUI outcome, including age, ovulation induction with HCG and – although only differing by trend –the presence of a male factor were confirmed; (ii) women with overt hypothyroidism who received thyroid hormone supplementation that was sufficient to keep TSH levels < 2.5 μIU/ml were not at greater risk of IUI failure; and (iii) for women who received thyroid hormone supplementation due to subclinical hypothyroidism, with TSH levels > 2.5 μIU/ml, higher pregnancy rates were found after IUI. Here, TG is linked to hypothyroidism.