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A recent clinical study has shown that treating pregnant women with levothyroxine, even if they are diagnosed with mild hypothyroidism, can offer the benefit of favourable obstetric outcomes at birth.

The thyroid gland is an endocrine organ that is located at the base of the neck and it produces hormones that are essential in controlling certain functions such as the body’s metabolic rate. Thyroid hormones are essential for the neurological development of the fetus, but these babies cannot produce their own thyroid hormones during the first trimester of pregnancy. They therefore have to rely on the mother's source of thyroid hormones until the second trimester when they do start producing their own hormones. 

Up to 3% of pregnant women have reduced levels of thyroid hormones and are therefore regarded as having mild hypothyroidism which can be treated with a synthetic replacement hormone called levothyroxine.

Reduced thyroid function in pregnancy is associated with negative obstetric outcomes, but it isn't clear whether treatment with levothyroxine, initiated during pregnancy, is beneficial for expectant mothers. A recent study that was done has made some relevant discoveries that will be discussed further in this article.

The study

Researchers from the University of Cardiff explored whether pregnant women who were diagnosed with mild hypothyroidism, together with their babies, would benefit from treatment with levothyroxine.

These researchers analyzed the clinical data of 13,224 women at 12-16 weeks of pregnancy. 518 of the expectant mothers had mild hypothyroidism, of which 263 of these women received the treatment levothyroxine and the rest received no medication. The women’s pregnancy outcomes were assessed by measuring data on preterm deliveries, stillbirth rates, fetal birth weights, the number of early caesarean sections that were performed and hospital length of stay.

The findings

The following findings were made in this study:

  • Women diagnosed with mild hypothyroidism, and who were treated with levothyroxine, had a decreased risk of giving birth to babies with a low weight and were also less likely to have an early caesarean section performed on them.
  • Women with mild hypothyroidism, who didn't receive treatment with levothyroxine, were more likely to have a stillbirth than women with a normal thyroid function.
  • The mentioned group that received treatment never experienced any stillbirths.
  • No significant difference between the other obstetric outcomes was found.
In summary, negative obstetric outcomes such as giving birth to babies with a low weight, stillbirths and early caesarean sections can be reduced by treating an extended number of pregnant women, diagnosed with mild hypothyroidism, with levothyroxine in order to increase the mother's thyroid hormone levels.

Clinical significance

Clearly, by treating pregnant women, even with mildly impaired thyroid functions, with thyroid supplementation can offer a better obstetric outcome for the unborn fetus. Essentially, this management helps to supplement the mother's thyroid hormones that are being utilised by the developing baby during the first trimester of pregnancy.

Thyroid screening is therefore essential in the ante-natal workup of a pregnant women in order to assess serum thyroid hormone levels and to decide if treatment is warranted in these patients.

Further research

It has been suggested that further clinical trials are needed to confirm the mentioned findings, and pregnant women may even discover more substantial benefits if they are treated with levothyroxine at an earlier stage than used in this study.

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