The effect of prolonged iodine deficiency on learning and motivation to achieve was studied in 100 male children selected from both severely iodine-deficient (SID) and mildly iodine-deficient (MID) villages and reported from the Department of Psychology, Kashi Vidyapeeth, Varanasi, and Department of Endocrinology, Banjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, India. Mean urinary iodine excretion and serum thyroxine concentration were significantly lower, and serum TSH was significantly higher, in the SID group than in the MID group. SID compared to MID children were slower learners, having lower scores on maze and pictorial learning tasks, and the SID group scored significantly lower on the achievement motivation scale. The SID and MID groups were matched for age, socioeconomic status, and formal education. Cretins were excluded, and study subjects were required to read and write. 
COMMENT. Iodine deficiency results in slow learning ability and impaired motivation to achieve. In severely iodine-deficient (SID) children, the number of errors and time taken in maze learning were negatively correlated with urinary iodine excretion and thyroxine concentrations, and showed a positive correlation with thyroid stimulating hormone. In contrast, tests involving verbal learning and free-recall, a less demanding cognitive task than maze learning, did not distinguish between severely and mildly iodine-deficient children, but SID children were slower on serial testing, showing impairment of formation of new stimulus-response associations or engrams. Optimal iodine nutrition is important in the prevention of learning disabilities and failure to achieve academically. The following report supports a correlation between neonatal triiodothyronine levels and later cognition.
Low triiodothyronine concentration in preterm infants was correlated with a mean 6.6 point deficit in overall IQ scores (8.5 deficit on verbal scale) on the WISC scales at 8-year follow up in a study of 236 infants examined at the Infant and Child Nutrition Group, Medical Research Council Dunn Nutrition Unit, Cambridge, UK . Brook CGD, of Cobbold Laboratories, Middlesex Hospital, London, in a Commentary: Do preterm infants need thyroxine replacement?  cautions that thyroxine therapy for hypothyroid mothers is unlikely to benefit premature babies, and triiodothyronine replacement in premature infants may be dangerous. Neonatologists generally withhold therapy pending a retest after the infant reaches term. An increased risk of developing cerebral palsy and cognitive deficits has been reported in premature infants with low thyroxine levels in the first week of life (N Engl J Med March 28 1996).
Iodine deficiency diseases (IDD), a major international public health problem especially affecting developing countries, may be prevented by the administration of iodinized salt. My colleague, Dr Charles Swisher, provided me with several references to the topic, pointing out that estimates have shown 200 million affected by IDD and 800 million people at risk worldwide for IDD, a total of “1 billion brains” at risk of maldevelopment or malfunction! The most serious complication of IDD is endemic cretinism, but milder forms of IDD may be associated with impaired cognitive function and learning disabilities in later childhood. For further reviews of IDD. [4, 5, 6, 7]