Risk of miscarriage
An excess of thyroid hormone during pregnancy has a direct toxic effect on the foetus.
University of Chicago researchers have showed, for the first time that an excess of thyroid hormone during pregnancy has a direct toxic effect on the fetus, tripling the risk of miscarriage and reducing the average birth weight of infants who survive.
"Although the risks of insufficient thyroid hormone during pregnancy are established, we have not, until now, been able to determine the consequences of an excess. We now see that having too much is just as bad as having too little. This tells us that hormonal replacement must be assessed and fine-tuned so as not to exceed the normal requirements," said Samuel Refetoff, the lead author of the study.
The study published in JAMA looked at pregnancy outcomes of 36 married couples from this extended family. Their sample included 18 couples in which either the mother or father was resistant to thyroid hormone and 18 unaffected couples.
They found that mothers with the mutation, who required higher levels of thyroid hormone to maintain a normal life, had a much higher rate of miscarriage, 23.7 per cent, about three times the rate for couples in which only the father had the mutation (6.7 per cent) or neither parent had the mutation (8.8 per cent).
Two-thirds (20 out of 31) shared this mutation and only one-third of the infants born to affected mothers did not. Unaffected mothers, with no increased rate of miscarriages, gave birth to equal numbers of affected and unaffected infants.
The mean birth weight of the unaffected infants born to affected mothers was 20 percent below average. Three of these infants had birth weights below the World Health Organization's criteria for low birth weight. Unaffected infants born to affected mothers also had extremely low levels of thyroid stimulating hormone (TSH), a definite sign of thyroid hormone excess.