BBC News reported that pregnant women who have pre-eclampsia are more likely to suffer from thyroid problems.
This news comes from well-conducted research that used two separate studies to explore whether pre-eclampsia during pregnancy affects thyroid function. Both studies found a clear link between pre-eclampsia and blood test results that indicated underactive thyroid function, but many questions remain unanswered. Principally, it is unclear whether these blood test results were associated with any noticeable health problems or later thyroid disease, and whether any thyroid problems persisted after the birth.
From this study, it is not possible to say whether pre-eclampsia raises the risk of thyroid problems or if thyroid problems contribute to pre-eclampsia. There is a need for further research into this association.
Where did the story come from?
This research was conducted by Richard Levine from the National Institute of Child Health and Human Development, Bethesda, USA, and colleagues from other institutions in the US and Norway. The study received funding from various sources, including the Eunice Kennedy Shriver National Institute of Child Health and Human Development, and through salary support from the National Institutes of Health in the US. The study was published in the peer-reviewedBritish Medical Journal.
Coverage by BBC News accurately reflected the study report, without going into great clinical detail.
What kind of research was this?
This research featured two different studies which explored the association between problems of the thyroid gland and pre-eclampsia. Pre-eclampsia is a condition in which a pregnant woman develops high blood pressure, fluid retention and protein in her urine. It raises the risk of further complications for both the mother and baby.
The first phase of the research was a case control study that compared women who had experienced pre-eclampsia during pregnancy with women that had not. The second phase looked at the results of a large cohort study in 7,121 women, which measured their thyroid function following their first pregnancy.
This well-conducted research used two study designs to investigate whether pre-eclampsia is associated with thyroid problems. It should be noted that the data for the first study phase was gathered from members of a previous trial that investigated a treatment to prevent pre-eclampsia (the Calcium for Pre-eclampsia Prevention trial). This study was not specifically designed to investigate the link between pre-eclampsia and thyroid problems, which adds a limitation to the case control part of this subsequent study. Also, the women who were selected for the study may have had specific characteristics which mean the observed associations may not apply to all pregnant women.
What did the research involve?
The calcium trial was conducted in the US between 1992 and 1995. It found that calcium supplementation did not have an effect on the risk of pre-eclampsia. The subsequent case control study matched 141 of the participants with pre-eclampsia (cases) with 141 women without the condition (controls). All had given blood samples before they developed pre-eclampsia, at around 21 weeks of pregnancy. Blood samples were also taken after pre-eclampsia started (just before delivery).
The researchers then looked for associations between underactive thyroid and levels of a blood enzyme that has been associated with pre-eclampsia (called soluble fms-like tyrosine kinase 1).
This case control study was limited by the fact that thyroid function was not measured after the birth. While the study measured levels of markers of thyroid function in the blood, it does not report whether the women experienced clinical signs and symptoms of an underactive thyroid during pregnancy or if symptoms persisted after the birth. This means that it is not possible to tell if the thyroid problems were harmful to the women or if any problems resolved themselves.
The cohort study phase was conducted in Norway between 1995 and 1997. It involved 7,121 women who had first given birth after 1967 and had subsequently had their thyroid function measured. The researchers used these records to calculate the risk of having an underactive thyroid in relation to having pre-eclampsia.
What were the basic results?
In the calcium trial, women who developed pre-eclampsia experienced a significant increase in levels of thyroid stimulating hormone (TSH) compared to control participants. They also experienced a decrease in their thyroid hormone levels. Together, these indicate an underactive thyroid in the women who developed pre-eclampsia.
Across both groups, increases in TSH concentration were significantly associated with increased levels of the kinase enzyme associated with pre-eclampsia.
In the cohort study phase, women with a history of pre-eclampsia in their first pregnancy had a significantly increased risk of having TSH levels that exceeded the normal range. In most of these cases, there was an absence of thyroid antibodies, which suggests that these levels were not due to autoimmune thyroiditis. This is the most common cause of an underactive thyroid and happens when the body’s immune system attacks its own thyroid cells.
It is unclear at what point following pregnancy the women had their thyroid function assessed and, therefore, how long the situation persisted. There is also no indication that thyroid function was associated with any disease symptoms, although the researchers’ indication that underactive thyroid was “subclinical” (without apparent symptoms) suggests that it was not.
How did the researchers interpret the results?
The researchers conclude that increased enzyme concentration in the blood during pre-eclampsia is associated with subclinical hypothyroidism (underactive thyroid) during pregnancy. They say that pre-eclampsia may also predispose women to having reduced thyroid function in later years.
This well-conducted research used two separate studies to examine whether pre-eclampsia during pregnancy affects thyroid function. Although both studies found a clear link between pre-eclampsia and blood tests that indicated an underactive thyroid, many questions remain unanswered.
- Principally, it is unclear whether blood test results that indicated an underactive thyroid were associated with any signs or symptoms of disease. While women had altered levels of one of the thyroid hormones tested, few women were found to have abnormal levels of both. Abnormal levels of just one hormone does not necessarily suggest clinical hypothyroidism.
- It is not known whether thyroid function went back to normal following birth, whether it persisted and for how long, or whether any hypothyroidism seen was severe enough to require treatment.
- The first case control study was not originally designed to investigate the association between pre-eclampsia and thyroid function. It was a trial to investigate the use of calcium during pregnancy, and the women selected for the study may not be representative of pregnant women in general.
- The cohort study phase only measured thyroid function following birth. It is not known how this compared to pre-pregnancy levels.
- It is not clear whether the health of the children in these studies was in any way affected.
The causes of pre-eclampsia are not known, although there may be a genetic link. There are various causes of an underactive thyroid, including problems of the immune system attacking the body’s thyroid tissue. Other causes include surgical treatment affecting the thyroid, iodine deficiency and side effects of some medications.
From this study, it is not possible to say whether pre-eclampsia contributes to underactive thyroid or if thyroid problems contribute to pre-eclampsia. It is also possible that a person with certain physiology would be more likely to develop both conditions. There is currently limited available evidence on the link between thyroid function and pre-eclampsia, and further research into this subject is needed.
Links to the headlines
Thyroid ‘risk from pre-eclampsia’. BBC news, November 17 2009
Links to the science
Levine RH, Vatten LJ, Horowitz GL et al.Pre-eclampsia, soluble fms-like tyrosine kinase 1, and the risk of reduced thyroid function: nested case-control and population based study. BMJ, November 17 2009 (published online)
Related review by Cochrane
Hofmeyr GJ, Atallah ÁN, Duley L. Calcium supplementation during pregnancy for preventing hypertensive disorders and related problems.