Why does the thyroid matter in fertility?

The thyroid is an endocrine gland which produces hormones that regulate the growth and function of other bodily systems. It is crucial for regulating metabolism, energy production, oxygen utilisation and hormone levels.

Thyroid hormones are produced by everybody, and play an important part in growth. As such they are critical in pregnancy. If the thyroid is over or underactive, this can affect ovulation regulation and reduce fertility. Many women suffer from thyroid dysfunction; around 25% of women in their lifetime, which is around 4x more women than men. Graves disease is the most common thyroid dysfunction, affecting 1% of the population. These conditions are important in fertility, because thyroid hormones are important in regulating ovulation, preventing miscarriage and aiding foetal brain development.

Thyroid hormone levels

Thyroid hormone levels can be measured by a simple thyroid stimulating hormone (TSH) blood test. It is important that TSH is at an optimal level, as it affects ovulation; up to 5% of women struggling to conceive have abnormal thyroid hormone levels. Thyroid hormone levels are also important in the development of the foetus, as they are crucial in growth. It has been found that children have higher IQs when expectant mothers are screened for thyroid stimulating hormone (TSH) levels, as potential thyroid dysfunctions are picked up and treated. The best way to assess thyroid levels is to ask for a blood test. Conventional wisdom suggests that 4.2 should be the upper limit for TSH. However, recent studies have suggested that TSH should be no higher than 2.5 when trying to conceive and 3.0 during pregnancy. Thyroid hormones are synthesised from iodine, so it is also important that iodine levels are optimal. If TSH or iodine levels prove to be too high/low, this can be easily rectified with supplementary medication.

Hyperthyroidism: Hyperthyroidism is when the thyroid is overactive and produces excess amounts of thyroid hormones. Graves disease and other autoimmune diseases are the most common manifestations of this. The presence of hyperthyroidism can be measured by performing a TSH blood test. Hyperthyroidism can cause infrequent periods in women and low sperm counts in men. Once hyperthyroidism has been diagnosed it can be treated with medication, radioactive iodine or surgery, after which hormone levels generally return to normal.

Hypothyroidism: Hypothyroidism occurs when the thyroid is underactive and does not produce enough thyroid hormones. This is often caused by a lack of iodine in the diet, or by autoimmune diseases such as Hasimoto’s thyroiditis. It is associated with reduced fertility, an increased risk of miscarriage, lower infant intelligence, pre-eclampsia, premature birth and infant death. Hypothyroidism can again be diagnosed with a simple blood test. Treatment is usually supplementation with iodine or a synthetic form of thyroid hormone, thyroxine.

Thyroid antibodies

As well as levels of thyroid hormones, thyroid antibodies are another factor which has an impact on fertility. These are produced if the thyroid is being mistakenly attacked by the immune system as part of an auto-immune disease, and they are present in between 8 and 30% of infertile women. The presence of thyroid antibodies increases the risk of pregnancy loss, making it about 4x more likely. The mechanism of this is not yet clear; it could affect implantation or interact with other antibodies. Again, the best way to pick up on this is to undergo a blood test, so that appropriate treatment can be arranged if necessary.

Preventative measures

Apart from diagnosis and medication, there are some lifestyle choices that can be taken to ensure that the thyroid functions properly. Reducing stress is important, as high cortisol (a stress hormone) levels inhibit thyroid hormone production. Increasing exercise is also helpful, as it promotes the production of thyroid hormones and increases the sensitivity of the tissues to these hormones. Modern diets also play a large part in thyroid dysfunction, in particular refined grains, sugars, soy products, peanuts and caffeine. Eating a healthy diet and avoiding smoking and alcohol consumption are therefore recommended.