Sera from nine species of clinically healthy nonhuman primates were assayed for T₄ and thyroid-stimulating hormone (TSH) using human immunoassays (a fluorescence polarization immunoassay for T₄, a microparticle enzyme immunoassay for TSH). The T₄ levels ranged from 20 to 132 nmol/L (x̄ ± SD = 62.8 ± 24.7 nmol/L). Levels of TSH were detected only in western lowland gorillas (Gorilla gorilla gorilla) and Sumatran orang-utans (Pongo pygmaeus abelii) (range, 0.4–10.8 mIU/L; detectable limit = 0.01 mIU/L). The results suggest that the antibodies used in the commercial TSH immunoassay assessed in this study cross-react with gorilla and orang-utan TSH but not with TSH of primates of the genera Macaca, Papio, Erythrocebus, Ateles, Leontopithecus, and Lemur.