Animal research has shown that within the visual system, thyroid hormone (TH) is necessary for the differentiation of cone photoreceptor subtypes. In humans, cones are responsible for colour vision and since it is possible that TH plays a similar role in human cone differentiation as it does in animals, it is therefore possible that children who experienced a lack of TH during development have colour vision deficits. In order to ascertain the role of TH in the development of human colour vision, the colour vision of children who experienced a TH deficiency during pregnancy or early life was assessed. Results indicated that children who experienced a third trimester or early life TH deficiency had deficits relative to controls on measures of the blue-yellow visual pathway. This deficit was related to some indices of TH levels during development and also to performance on behavioural tasks where colour was a salient feature.