The frequent observation that TSH levels are elevated during obesity is not regarded as a functional defect, but as a mere manifestation of a deranged hypothalamic–pituitary axis or an increase in leptin-induced TRH production and type 2 iodothyronine deiodinase (D2) inhibition in the thyrotrophs [28]. Here, TRH is linked to obesity due to melanocortin 4 receptor deficiency.