Germline loss-of-function mutations of TSH receptor (TSHR) gene have been described in families with partial or complete TSH resistance. Large TSH elevations were generally found in the patients with homozygous or compound heterozygous mutations. In this study, we sequenced the entire TSHR gene in a series of 10 unrelated patients with slight (6.6-14.9 mU/liter) to moderate (24-46 mU/liter) elevations of serum TSH, associated with definitely normal free thyroid hormone concentrations. Thyroid volume was normal in all patients, except two with a modest hypoplasia. Autoimmune thyroid disease was excluded in all patients on the basis of clinical and biochemical parameters. Eight patients had at least one first-degree relative bearing the same biochemical picture. TSHR mutations were detected in 4 of 10 cases by analyzing DNA from peripheral leukocytes. A compound heterozygosity (P162A on maternal allele, and the novel mutation C600R on the paternal one) was found in the patient with the highest TSH levels. Only one TSHR allele was mutated in the remaining three cases, and no alterations in TSHR gene promoter were detected in all of these probands. A novel mutation (L467P) was detected on the maternal allele in one patient and in her monozygotic twin. Previously described inactive mutants, T655Delta and C41S, were detected in the other two cases. When tested on several occasions, circulating TSH values fluctuating above the upper limit of the normal range could be shown in heterozygous subjects of these families. A dominant mode of inheritance of the biochemical alterations was detected in these cases. Mutant TSHRs were studied during transient expression in COS7 and HEK293T cells. Their TSH-independent cAMP accumulation activities were very low or similar to mock-transfected cells, and no increases were seen after maximal hormone stimulation. Flow cytometry experiments showed a poor level of expression of all mutant TSHRs at the cell membrane. In conclusion, we found several loss-of-function mutations of TSHR, including two novel ones, in a series of unrelated patients with slightly elevated TSH levels. Therefore, partial resistance to TSH action is a frequent finding among patients with slight hyperthyrotropinemia of nonautoimmune origin. Germline mutations of TSHR may be associated with serum TSH values fluctuating above the upper limit of the normal range, also in the heterozygous state.