Constitutively-activating germline mutations of the thyrotropin receptor (TSHR) gene are very rare and are considered the cause of hereditary nonautoimmune hyperthyroidism. We describe four affected individuals from a Caucasian family: a mother and her three children, and an unaffected father. The mother and her first two children presented in a similar manner: lifelong histories of heat intolerance, hyperactivity, fast heart rate, reduced energy, increased appetite, and scrawny build. They all developed goiter in childhood and showed a suppressed TSH and elevated thyroxine (T(4)). The last child, a 12-year-old female, presented with no clinical symptoms or palpable neck mass, but with a suppressed TSH, elevated T(4) and thyromegaly detected by ultrasound. Mutation analysis of the TSHR gene in all family members revealed a novel heterozygous germline mutation resulting in the substitution of phenylalanine (TTC) by serine (TCC) at codon 631 in transmembrane helix 6 in the mother and all three children. Functional characterization of this germline mutation showed constitutive activation of the G(s)-mediated cyclic adenosine monophosphate (cAMP) pathway, which controls thyroid hormone production and thyroid growth. Molecular characterization of F631S demonstrates that this activating mutation plays a key role in the development of hereditary hyperthyroidism in this family although the timing of onset of clinical manifestations in the subjects may depend on other, as yet unidentified, factors.