An excess of thyroid hormone during pregnancy can harm the unborn baby, warn Chicago University researchers.
Women receiving replacement therapy should not be alarmed
But women taking replacements should not worry unduly say UK experts.
High levels tripled miscarriage risk and hampered weight gain in babies who did survive, the US team report in Journal of the American Medical Association.
Replacement is recommended for pregnant women lacking sufficient levels, but it should be tightly controlled they say.
The problems caused by a lack of thyroid hormone during pregnancy, such as miscarriage, are well known.
It can also result in mental retardation in the child.
Women with underactive thyroids, a condition called hypothyroidism, are advised to take supplements to replace the shortage.
A fine balance
But for the first time, researchers have shown overcompensating for this shortage could be equally harmful to the baby.
Dr Samuel Refetoff and colleagues studied people with a rare genetic disorder that makes them resistant to thyroid hormone.
This means even though their levels of the hormone are abnormally high they are unaffected by it and have none of the usual symptoms, such as anxiety and excessive weight loss.
Affected mothers and fathers only pass this trait on to half of their children.
Mothers with the disorder will expose their offspring to high levels of the hormone while in the womb. This will not matter if the baby also has the trait because they will be resistant to the high levels, but it will if they baby does not inherit the disorder.
Conversely, babies born to healthy mothers who inherit the trait from their father will experience less than optimal hormone levels despite levels in the womb being normal.
The researchers compared the pregnancy outcomes of 18 couples in which either the mother or father had the genetic trait with the pregnancy outcomes of 18 healthy couples.
Mothers with the genetic disorder were three times more likely to miscarry than those without the disorder, regardless of whether the father had the disorder or not.
Although it would be expected that half of the children born to affected mothers would have the mutation, two-thirds did.
The researchers said this suggests healthy babies are susceptible to high levels of thyroid hormone and the genetic disorder protects babies who inherit the trait from the affects of this hormone excess.
The average weight of the unaffected babies born to mothers with the trait was 20% below average.
Dr Refetoff said: "We now see that having too much is just as bad as having too little.
"This tells us that hormone replacement must be assessed and fine-tuned so as not to exceed the normal requirements," he said.
Dr Prakash Abraham, assistant secretary of the British Thyroid Association and honorary senior lecturer at the University of Aberdeen, said women on thyroid hormone replacement therapy should not be alarmed at the findings.
"I would not be alarmist about it. These women studied were a small group and, because of their genetic background, I suspect they are exposed to much higher levels of hormone than would be given to women treated with thyroid hormone for hypothyroidism.
"I agree that we should be aware of the possibility that over-replacement is detrimental, but everyone strives to keep things within the normal range during pregnancy and women who are hypothyroid and given hormone treatment are monitored closely," he said.
Professor Lucilla Poston from the baby charity Tommy's said: "This study offers some interesting information which infers, but does not directly prove, that high levels of thyroid hormone in the mother may be damaging to the fetus and cause miscarriage.
"However, the numbers in the study are small and all subjects were members of the same family who have a rare genetic disorder, so it cannot be ruled out that other common genetic influences within the family may play a role."