Mutations in the thyroid hormone receptor gene (TR) cause resistance to thyroid hormone (RTH). How the action of mutant thyroid hormone nuclear receptors (TRs) is regulated in vivo is not clear. We examined the effect of a TR coactivator, steroid receptor coactivator-1 (SRC-1), on target-tissue responsiveness by using a mouse model of RTH, TRPV knockin mice, in the SRC-1 null background. Lack of SRC-1 intensified the dysfunction of the pituitary-thyroid axis and impaired growth in TR^PV/+ mice, but not in TR^PV/PV mice. In TR^PV/PV mice, however, lack of SRC-1 intensified the pathological progression of thyroid follicular cells to papillary hyperplasia, reminiscent of papillary neoplasia. In contrast, lack of SRC-1 did not affect responsiveness in the liver in regulating serum cholesterol in either TR^PV/+ or TR^PV/PV mice. Lack of SRC-1 led to changes in the abnormal expression patterns of several T3 target genes in the pituitary and liver. Thus the present studies show that a coactivator such as SRC-1 could modulate the in vivo action of TR mutants in a tissue-dependent manner.