SummaryObjective  Genetic analysis of the TSH receptor gene in seven subjects with subclinical hypothyroidism (SH), in whom the diagnosis of autoimmune thyroid disease had been excluded by laboratory and instrumental techniques currently available.Patients  Three families where different members (2 children and 5 adults) affected by SH were studied.Genetic analysis  Genomic DNA was extracted from peripheral lymphocytes and the entire coding sequence of the TSHr gene was sequenced. pSVL‐TSHr construct harbouring a Q8fsX62 insertion was obtained by site‐directed mutagenesis. COS‐7 cells transfected with wild‐type and mutant receptor were used for binding studies, flow cytometry, and cyclic AMP (cAMP) determination.Results  A four base pair (bp) duplication in position 41 (41TGCAins), leading to a premature stop of translation at codon 62 (Q8fsX62), was found to be heterozygous in the proband, the father and the sister in Family 1. In Family 2 the proband and the sister were heterozygous for the mutation D410N. In Family 3 the proband and the father were heterozygous for the mutation P162A. After transfection in COS‐7 cells, the mutant receptor Q8fsX62 displayed a low expression at the cell surface, and a reduced response to bovine TSH (bTSH) in terms of cAMP production.Conclusions  We identified TSH receptor mutations in seven members of three families with subclinical hypothyroidism.