Relation of severity of maternal hypothyroidism to cognitive development of offspring

An association between maternal subclinical hypothyroidism and low intelligence quotient (IQ) in the offspring has recently been shown.

To provide evidence for the causality of the association by testing the hypothesis that severity of maternal hypothyroidism correlates inversely with IQ of the offspring.

IQ scores were compared among 8 year old offspring of 124 control mothers whose thyroid stimulating hormone (TSH) concentrations were < 98th percentile of a cohort of 25,000 mothers at 17 weeks gestation, of 28 untreated hypothyroid women whose TSH was between the 98th and 99.85th percentiles, and of 20 untreated women whose TSH concentration was > or = 99.85th percentile.

Mean (SD) IQs for each group of children (in ascending order of maternal TSH concentration) were 107 (13), 102 (15), and 97 (14). The difference between the extremes was significant (p = 0.003). The percentage of children with IQs > 1 SD below the control mean was 15, 21, and 50 respectively (p = 0.003). The odds ratio of having an IQ > 1 SD below the control mean, after controlling for socioeconomic status, was 4.7 (p = 0.006) for the third group compared with the controls.

The inverse correlation between severity of maternal hypothyroidism and IQ of the offspring supports a causal relation and makes the need to screen for and treat pregnant women for hypothyroidism even more compelling.