Objective To analyze the reference range of thyroid‐stimulating hormone (TSH) in different BMI categories and its impact on the classification of hypothyroidism. Methods The study included 3,928 individuals free of thyroid disease (without previous thyroid disease, no interfering medications, TSH <10 µUI/mL and thyroid peroxidase antibodies TPO Abs <50 IU/mL) who participated in a national, cross‐sectional, population‐based study and were representative of the adult population of Spain. Data gathered included clinical and demographic characteristics, physical examination, and blood and urine sampling. TSH, free thyroxine, free triiodothyronine, and TPO Ab were analyzed by electrochemiluminescence (E170, Roche Diagnostics, Basel, Switzerland). Results The reference range (p2.5‐97.5) for TSH was estimated as 0.6 to 4.8 µUI/mL in the underweight category (BMI<20 kg/m2), 0.6 to 5.5 µUI/mL in the normal‐weight category (BMI 20‐24.9 kg/m2), 0.6 to 5.5 µUI/mL in the overweight category (BMI 25‐29.9 kg/m2), 0.5 to 5.9 µUI/mL in the obesity category (BMI 30‐39.9 kg/m2), and 0.7 to 7.5 µUI/mL in the morbid obesity category (BMI ≥40). By using the reference criteria for the normal‐weight population, the prevalence of high TSH levels increased threefold in the morbid obesity category (P < 0.01). Conclusions Persons with morbid obesity might be inappropriately classified if the standard ranges of normality of TSH for the normal‐weight population are applied to them.