Review question
Does hormone supplementation with thyroxine (levothyroxine) improve fertility outcomes after in vitro fertilisation (a fertility treatment where an egg is combined with sperm outside the body) or intracytoplasmic sperm injection (a fertility treatment where a single sperm is injected directly into an egg) for women diagnosed with presence of thyroid antibodies (autoimmune thyroid disease; ATD) or mildly underactive thyroid?
Background
Thyroid disease is the second most common hormonal disorder affecting women of reproductive age. Research has shown a higher rate of miscarriage and reduced fertility in women with underactive (slow-working) thyroid, with both thyroid hormones measured in blood testing being low. However, there is also a mild variant of this thyroid disease, the so-called 'subclinical' or mildly underactive thyroid in which affected people show no symptoms and only have a mild change in one of the thyroid hormones when testing blood levels. Another mild variant of thyroid disease is the so-called 'ATD,' with normal thyroid hormone levels, but the presence of thyroid antibodies. Antibodies could attack a woman's own body cells, and the presence of thyroid antibodies is associated with a higher risk of miscarriage. To date, it is unclear what these mild subtypes of thyroid disease do to female fertility and pregnancy outcomes.
Study characteristics
Cochrane authors performed a comprehensive literature search of the standard medical databases to 8 April 2019 in consultation with the Cochrane Gynaecology and Fertility Group Information Specialist, for randomised clinical trials (RCTs: clinical studies where people are randomly put into one of two or more treatment groups) investigating the effect of thyroid hormones (levothyroxine) for women diagnosed with ATD or mildly underactive thyroid who were planning to undergo assisted reproduction. Two authors independently selected studies, evaluated them, extracted data and attempted to contact the authors where data were missing.
We found four RCTs (with 820 women) that met our inclusion requirements. The thyroid hormones were administered in a range of doses to women diagnosed with mildly underactive thyroid or presence of thyroid antibodies (ATD).
Key results
In women with mild thyroid hormone imbalance and unknown thyroid autoimmunity status, we were uncertain whether thyroxine replacement had an effect on live birth or miscarriage rates (very low-quality evidence from one study involving 70 women).
In women with mildly underactive thyroids (with or without ATD), the evidence suggested that thyroxine replacement may have improved live birth rates (low-quality evidence from one study involving 64 women) and it may have led to similar miscarriage rates (low-quality evidence from one study involving 64 women). The evidence suggested that women with mildly underactive thyroid (with or without ATD) would have a 25% chance of a live birth with placebo or no treatment, and 27% to 100% with thyroxine.
In women with ATD and normal thyroid function, treatment with thyroxine replacement compared with placebo or no treatment may have led to similar live birth rates (low-quality evidence from two studies involving 686 women) and miscarriage rates (low quality evidence from two studies involving 686 women). The evidence suggested that women with ATD and normal thyroid function would have a 31% chance of a live birth with placebo or no treatment, and 26% to 40% with thyroxine.
Side effects were rarely reported. One study reported none out of 32 preterm births in the thyroxine replacement group and one out of 32 preterm births in the control group in women diagnosed with mildly underactive thyroid (with or without ATD). One study reported 21 out of 300 preterm births in the thyroxine replacement group and 19 out of 300 preterm births in the control group in women diagnosed with ATD and normal thyroid function. None of the studies reported on other maternal pregnancy complications, foetal complications or side effects of thyroxine.
Quality of the evidence
The evidence was of very low to low quality. We downgraded the evidence as it was based on single, small trials with widely variable results.
We could draw no clear conclusions in this systematic review due to the very low to low quality of the evidence reported.
Thyroid disease is the second most common endocrine disorder affecting women of reproductive age. Subclinical hypothyroidism is diagnosed by an elevated thyroid-stimulating hormone concentration with a normal concentration of free thyroxine hormone. Autoimmune thyroid disease (ATD) is diagnosed by the presence of thyroid autoantibodies, regardless of thyroid hormone levels. Thyroxine may be a useful treatment for subfertile women with these two specific types of thyroid disease for improving pregnancy outcomes during assisted reproduction.
To evaluate the efficacy and harms of levothyroxine replacement in subfertile women with subclinical hypothyroidism or with normal thyroid function and thyroid autoimmunity (euthyroid autoimmune thyroid disease, or euthyroid ATD) undergoing assisted reproduction.
We searched the Cochrane Gynaecology and Fertility (CGF) Group specialised register, CENTRAL, MEDLINE, Embase, PsycINFO, CINAHL and two trials registers together with reference checking and contact with study authors and experts in the field to identify studies. We searched for all published and unpublished randomised controlled trials (RCTs) comparing thyroxine with no treatment or placebo, without language restrictions, from inception to 8 April 2019, and in consultation with the Cochrane CGF Information Specialist.
We included women undergoing assisted reproduction treatment, meaning both in vitro fertilisation and intracytoplasmic sperm injection, with a history of subfertility and with subclinical hypothyroidism or with euthyroid ATD. We excluded women with a previously known clinical hypothyroidism or already taking thyroxine or tri-iodothyronine. RCTs compared thyroxine (levothyroxine) with either placebo or no treatment.
We used standard methodological procedures expected by Cochrane. Our primary review outcomes were live birth and adverse events of thyroxine; our secondary outcomes were clinical pregnancy, multiple pregnancy and miscarriage.
The review included four studies with 820 women. The included studies were of overall low risk of bias. Using GRADE methodology, we assessed the quality of evidence for the primary outcomes of this review to be very low- to low-quality evidence. Evidence was downgraded for imprecision as it was based on single, small trials with wide confidence intervals (CI). We were able to include data from three of the four included studies.
In one study of women with both subclinical hypothyroidism and positive or negative anti-TPO antibodies (autoimmune disease), the evidence suggested that thyroxine replacement may have improved live birth rate (RR 2.13, 95% CI 1.07 to 4.21; 1 RCT, n = 64; low-quality evidence) and it may have led to similar miscarriage rates (RR 0.11, 95% CI 0.01 to 1.98; 1 RCT, n = 64; low-quality evidence). The evidence suggested that women with both subclinical hypothyroidism and positive or negative anti-TPO antibodies would have a 25% chance of a live birth with placebo or no treatment, and that the chance of a live birth in these women using thyroxine would be between 27% and 100%.
In women with normal thyroid function and thyroid autoimmunity (euthyroid ATD), treatment with thyroxine replacement compared with placebo or no treatment may have led to similar live birth rates (risk ratio (RR) 1.04, 95% CI 0.83 to 1.29; 2 RCTs, number of participants (n) = 686; I2 = 46%; low-quality evidence) and miscarriage rates (RR 0.83, 95% CI 0.47 to 1.46, 2 RCTs, n = 686, I2 = 0%; low-quality evidence). The evidence suggested that women with normal thyroid function and thyroid autoimmunity would have a 31% chance of a live birth with placebo or no treatment, and that the chance of a live birth in these women using thyroxine would be between 26% and 40%.
Adverse events were rarely reported. One RCT reported 0/32 in the thyroxine replacement group and 1/32 preterm births in the control group in women diagnosed with subclinical hypothyroidism and positive or negative anti-TPO antibodies. One RCT reported 21/300 preterm births in the thyroxine replacement group and 19/300 preterm births in the control group in women diagnosed with positive anti-TPO antibodies. None of the RCTs reported on other maternal pregnancy complications, foetal complications or adverse effects of thyroxine.