The chapter on childhood ailments in 'The Hygiene of Life and Safer Motherhood', published in London in the 1930s has an entry on "Cretinism".
"Cretin is the name given to a child in whom the thyroid gland is absent, or has not developed. The thyroid gland consists of two oval bodies, one lying on each side of the windpipe in the neck." This gland secretes blood which is necessary for normal growth in a child. "When this secretion is absent, the growth of the bones practically ceases. The child lies unnaturally quiet in his cot, and makes few of the little movements we look for in a normal baby. He is unresponsive, and shows little inclination to do many of the things that ordinary babies take a pleasure in doing. His speech has been long in coming and he learns with difficulty to say a few single words. He is late in learning to use his hands, in sitting up, standing and walking, and learning clean habits. He scarcely ever laughs and rarely cries in a good healthy way. In most caes he will have a habit of keeping his tongue protruded between his lips."
Treatment involves giving the child thyroid extract regularly for several years.
It is possible that some autistic children were labelled as cretins.
The entry on mental deficiency describes it as a failure of intelligence. It ranges "from those cases where the child is unable to acquire the simplest control or thinking power, right up to those children who just fail to reach the normal standard of intelligence."
Special classes and special schools provide opportunities for developing these children's latent potential, and all their brain power can be utilised to make their lives happy, and if possible useful.
There is no mention in the book of the terms 'idiot, 'imbecile' or 'moron' which were contemporary terms for mentally ret*d people/people with learning difficulties.