Thyroid hormone receptors (TR) play critical roles in virtually all tissues. The TR ligand-binding domain (LBD) participates in important activities, such as transcriptional activation and repression, through conformational changes induced by hormone binding. Two crystal forms of isoform alpha1 of the human thyroid hormone receptor LBD (hTRalpha1) in complex with the thyroid hormones T3 and Triac were obtained. The hTRalpha1-T3 complex was crystallized in a previously unobserved crystal form (space group P2(1)2(1)2(1), a = 59.98, b = 80.80, c = 102.21 A), with diffraction patterns extending to 1.90 A resolution on a rotating-anode X-ray source, and in space group C2 (a = 117.54, b = 80.66, c = 62.55 A, beta = 121.04 degrees), with data extending to 2.32 A resolution. The hTRalpha1-Triac complex was also crystallized in the new space group P2(1)2(1)2(1), with unit-cell parameters a = 60.01, b = 80.82, c = 102.39 A; its resolution limit extended to 2.20 A on a home source. Phasing was carried out by the molecular-replacement method and structural refinement is currently in progress. The refined structures may provide insight into the design of new thyromimetics.