A new autosomal recessive mutation that causes hypothyroidism has been identified in mice. The gene, herein named hypothyroid ( hyt ), has been mapped on chromosome 12 approximately 30 units from the centromere. The mutants are characterized by retarded growth, infertility, mild anemia, elevated serum cholesterol, very low to undetectable serum thyroxine, and elevated serum thyroid-stimulating hormone. Thyroid glands are in the normal location but are reduced in size and hypoplastic. Mutant mice respond to thyroid hormone therapy by improved growth and fertility. These findings suggest that the hyt mutant gene results in primary hypothyroidism unresponsive to thyroid-stimulating hormone.