blood tests
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Thyroid health and female fertility: why blood testing matters
It’s not a hormone most people think about when planning for pregnancy, but the thyroid plays a quiet, important role in fertility. Nestled in the neck, this small gland affects everything from energy levels to menstrual cycles. And when its signals are off, even slightly, it can impact a woman’s ability to conceive or carry a pregnancy to term.
The thyroid’s influence on reproduction
The thyroid produces two main hormones:1
Thyroxine (T4)
Triiodothyronine (T3)
These regulate the body’s metabolism, but they also help control the development and function of the ovaries, uterus, and placenta. Too little or too much can disrupt the delicate hormonal rhythm needed for ovulation, embryo implantation, and healthy foetal growth.1,2
Women with underactive thyroids (hypothyroidism) may notice irregular or absent periods, difficulty conceiving, or an increased risk of miscarriage. Overactive thyroids (hyperthyroidism), while less common, can also cause cycle disruption and pregnancy complications.1
Why it’s not always obvious
One of the biggest challenges is that thyroid problems don’t always announce themselves. Symptoms like tiredness, weight changes, mood swings, and irregular periods are easy to blame on everyday life. That’s why many thyroid issues go undiagnosed, especially when someone isn’t actively looking for them.3
In fertility assessments, thyroid health is sometimes overlooked. But it shouldn’t be. Even mild thyroid dysfunction can affect reproductive hormones and the lining of the womb. Left unaddressed, it may lead to subfertility, miscarriage, or complications later in pregnancy.1
The case for blood testing
A simple blood test can measure thyroid-stimulating hormone (TSH), which signals whether the thyroid is overworking or underperforming. Most tests also include free T4 and sometimes free T3, which are the actual thyroid hormones circulating in the blood.1
Other helpful markers include thyroid peroxidase (TPO) antibodies, which can detect autoimmune thyroid conditions such as Hashimoto’s disease. These may cause the thyroid to slow down over time, even if blood levels look “normal” in the early stages.1
If a woman is trying to conceive, or has had difficulty in the past, checking these markers can offer useful insight, and often, a path forward. Mild thyroid dysfunction can often be managed with lifestyle changes or low-dose prescription medication, improving the chances of a successful pregnancy.
What the markers mean
TSH (thyroid-stimulating hormone): High TSH often means an underactive thyroid. Low TSH may point to overactivity.
Free T4 and Free T3: These show how much active thyroid hormone is available. They help confirm whether TSH levels are translating into normal thyroid function.
TPO antibodies: A marker for autoimmune thyroid disease, even when TSH and T4 are still within range.
It’s important to note that there isn’t a specified range used specifically with fertility in mind. It would be a specialist who would be able to interpret thyroid levels on a deeper level should they think that their patient’s fertility is being affected by their thyroid.1
Thyroid health in pregnancy
Thyroid hormones continue to matter long after conception. During the first trimester, the baby relies entirely on the mother’s thyroid hormones for brain and organ development. If those hormone levels are too low, it may affect the baby’s growth.2
Pregnancy itself puts extra strain on the thyroid. Hormonal changes, increased blood volume, and the placenta all shift thyroid demands, making it even more important to monitor levels early and often if there’s a known thyroid issue.2
The numan take
Fertility is complex, and there are many moving parts. But for women struggling to conceive or trying to understand changes in their cycle, the thyroid is worth a closer look. A quick blood test could provide answers, offer reassurance, or help guide the next step. It’s a small action that can make a big difference.
References
Silva JF, Ocarino NM, Serakides R. Thyroid hormones and female reproduction. Biology of reproduction. 2018;99(5): 907–921.
Singh S, Haq N, Sandhu S. Thyroid disease and pregnancy. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025.
Gottwald-Hostalek U, Schulte B. Low awareness and under-diagnosis of hypothyroidism. Current medical research and opinion. 2022;38(1): 59–64.
Written by Hassan Thwaini
Clinical Pharmacist and Copywriter, Master of Pharmacy (MPharm)
