A gonadal stromal neoplasm composed only of theca cells, occurring mostly in the postmenopausal ovary. It is filled with lipid-containing spindle cells and produces estrogens that can lead to endometrial hyperplasia; uterine hemorrhage; or other malignancies in postmenopausal women and sexual precocity in girls. When tumors containing theca cells also contain fibroblasts, they are identified as thecoma-fibroma tumors with less active hormone production.