Most people would be forgiven for assuming that a woman's ability to fall pregnant is determined by what goes on in her reproductive organs. But there’s an important group of hormones that have a far greater influence on fertility than they're given credit for. We should be looking for clues not down below, but in the neck - at the thyroid.
This small, butterfly-shaped gland that sits just behind the Adam's apple is important for female reproduction, both before conception and throughout pregnancy.
Your thyroid gland
The thyroid gland produces two hormones: thyroxine (sometimes called T4) and triiodothyronine (T3). Among other things, the thyroid hormones regulate metabolism.
The most common thyroid disorder is an underactive thyroid gland, known as hypothyroidism, where the gland doesn't produce enough thyroxine. It's usually diagnosed after an abnormally high reading on a thyroid-stimulating hormone (TSH) blood test.
Typically, hypothyroidism manifests in quite vague symptoms, like tiredness, depression, weight gain and sensitivity to cold; generally, people feel a bit run down, as though things have just slowed down, and this can have an indirect affect on reproduction.
"An underactive thyroid gland makes everything in the body a bit more sluggish, and that includes the way the ovaries work, and probably the health of the very early pregnancy," explains Dr Karen Morton, consultant gynaecologist and obstetrician for Dr Morton's - the medical helpline.
But the link between fertility and thyroid function is, in fact, more direct than many people think.
Thyroid health and fertility
Hypothyroidism can interact with other important processes that are responsible for making and releasing two crucial reproductive hormones: follicle-stimulating hormone (FSH) and luteinising hormone (LH).
In women, FSH - as the name suggests - stimulates the follicles of the ovary to grow in preparation for releasing an egg, while LH helps to regulate the menstrual cycle and ovulation. An imbalance of these hormones affects ovulation and is a major cause of female fertility problems.
Hypothyroidism can have a direct effect on these hormones: a deficiency in thyroxine leads to lower levels of FSH and LH in the blood.
Some studies have found that subfertile women - those who haven't conceived after a year of regular, unprotected sex - tend to have higher levels of TSH and increased rates of hypothyroidism than women who conceive without difficulty. In addition, pregnancy rates tend to be lower in women with raised TSH levels.
Once a woman falls pregnant, the thyroid's job continues by contributing to the normal growth of the baby in a number of ways, including the transfer of crucial thyroxine from the mother to the baby through the placenta.
"Thyroid hormones affect the growth of your baby's brain early on during the pregnancy," adds Simon Pearce, professor of endocrinology at the University of Newcastle. "Abnormal circulating thyroid hormone levels in the mother during pregnancy are associated with poor pregnancy outcomes such as miscarriage."
When should treatment be considered?
Thankfully, hypothyroidism can be treated very simply, by taking daily medication called levothyroxine that replaces the thyroxine (T4) hormone your body isn't producing sufficiently. The problem is that often, subfertile women don't know they might have an underactive thyroid gland, and many doctors don't look for it.
Dr Jane Stewart, secretary of the British Fertility Society, explains that when hypothyroidism has the effect of reducing fertility, physical symptoms will typically be present - and it's those symptoms that prompt doctors to investigate thyroid function.
"Hypothyroidism may result in menstrual irregularities and reduced fertility because of reduced ovulation [but] it would usually be clinically evident, and if appropriately replaced those effects would be reversed," she says. "Consequently, screening for thyroid problems for fertility purposes is not recommended."
However, as Professor Pearce highlights, symptoms of hypothyroidism are nonspecific, usually develop over several years and could indicate any number of other medical problems. "One study showed that 20% of people with fully normal thyroid blood tests had four or more symptoms of thyroid underactivity," he says.
As such, he suggests that doctors adopt a strategy of "aggressive case finding" when a woman is having unexplained fertility problems.
That means if you've had any hint of a thyroid problem before, if anyone in your family has had a thyroid problem, or if you have a disease that puts you at risk of a thyroid problem - for instance type 1 diabetes or coeliac disease - then you should definitely have a thyroid blood test before pregnancy."
Women with a history of miscarriage or any other obstetric problem should also be investigated, he says.
Optimising thyroid health for conception and beyond
Women with diagnosed hypothyroidism who are planning a pregnancy should have their thyroid hormone and TSH levels checked and, if required, their treatment adjusted.
"We like the thyroxine level to be a bit higher during pregnancy than in the non-pregnant situation, and the same applies with trying to get pregnant," says Dr Morton.
She also recommends all women have a thyroid function test if they've been trying to get pregnant for more than six months, even though the NHS does not endorse routine fertility-related thyroid testing.
A 2015 study makes the case for universal thyroid screening for women who are subfertile or experience recurrent early pregnancy loss, concluding that the risks of "significant effects on reproduction from to conception to birth ... can be significantly reduced if not ameliorated".
That may be a way off, but women struggling to get pregnant should adopt a healthy diet and lifestyle - whether or not they have or suspect a thyroid problem. And if they do, a good starting point is to talk with their GP.