Underactive Thyroid (Hypothyroidism) and Fertility
An underactive thyroid is very common in women. Around 7% of women in the UK are affected, and Hashimoto’s disease accounts for roughly 90% of all cases. An underactive thyroid and fertility are deeply connected, especially in the earliest stages of conception. Many women only discover a thyroid issue when they start trying to conceive — or after months of feeling unheard, exhausted, or “not quite right.”
Here, we explore how an underactive thyroid affects fertility, early pregnancy, and your ability to carry to term — and the gentle steps that can help create a safer environment for conception.
Symptoms of an Underactive Thyroid
Thyroid symptoms overlap with many other conditions. That overlap is one reason women are often dismissed or left without clear answers. A full thyroid panel is the best way to understand what is happening.
Common symptoms include
Fatigue or deep exhaustion
Unexplained weight gain
Brain fog or memory issues
Sensitivity to cold
Thinning or straw-like hair
Slow-growing or brittle nails
Dry skin
Aches or joint pain
Constipation
Irregular or heavy periods
Hashimoto’s and Autoimmune Thyroid Disease
Most cases of an underactive thyroid are autoimmune. Hashimoto’s thyroiditis is when the immune system begins targeting thyroid tissue, which slowly reduces the gland’s ability to produce hormone.
Autoimmune thyroid disease develops when several underlying factors place the immune system under strain. These may include chronic stress and HPA-axis dysfunction, nutrient insufficiency, gut dysbiosis or increased gut permeability, past infections, environmental triggers, and genetic susceptibility.
Autoimmunity usually develops over time. It is rarely caused by a single factor. Instead, it is a combination of stress physiology, inflammation, nutrient gaps, and immune sensitivity that leads to reduced thyroid hormone production.
Thyroid Tests: What May Be Missing
Standard thyroid testing often includes TSH and FT4. These measure thyroid function but do not check for autoimmune activity.
Thyroid antibodies — TPO antibodies and Tg antibodies — are not routinely included in testing in Ireland unless the doctor requests them. If you are trying to conceive, you may need to ask your doctor to include these antibody tests to check for autoimmune thyroid disease.
Current HSE guidance notes that thyroid antibodies are not routinely included unless specifically requested
It is possible to have raised TPO or Tg antibodies even when TSH, FT4 and FT3 appear within the normal range. Autoimmune activity can still affect fertility by increasing inflammation, altering ovulation signals, and reducing the availability of thyroid hormone in early pregnancy. This is why thyroid antibodies matter, even when standard bloods look “fine”.
Medication and What It Supports
Eltroxin (levothyroxine) is the standard medication used in Ireland for underactive thyroid. It replaces T4, the hormone the thyroid can no longer produce in adequate amounts.
Medication helps correct low thyroid hormone, which supports energy, mood, and early pregnancy development. However, it does not reduce autoimmune activity or treat the underlying triggers of Hashimoto’s.
This is why some women see their dose increase over time. This may be due to increased physiological demand, reduced thyroid tissue, or reduced conversion of T4 to T3 during times of stress, illness, or nutrient deficiency.
Underactive Thyroid and Fertility
Thyroid hormones are central to fertility. They support ovulation, progesterone production, implantation, and embryo development. During the earliest stages of pregnancy, the embryo depends heavily on maternal thyroid hormone.
Low availability of thyroid hormone can increase the risk of irregular cycles, anovulation, and miscarriage. Thyroid hormone sits at “Level 4” in the hormone hierarchy, meaning it depends on stable blood sugar, stress physiology, and nutrient sufficiency to function well.
Many women are prescribed a low dose of Eltroxin when trying to conceive, even if they are only borderline. This supports hormone availability for early development but does not address the deeper contributors such as inflammation, nutrient gaps, or stress load.
How Nutrition and Lifestyle Support Thyroid Hormone Availability
Nutrition and lifestyle do not treat autoimmune thyroid disease. But they do support the environment in which thyroid hormones are produced, converted, and used — which directly influences fertility.
Key areas include
Stable blood sugar, which reduces inflammation and supports hormone signalling
Sufficient protein and omega-3s to support hormone production
Nutrients such as selenium, zinc, iron, vitamin D and B vitamins, which support T4 to T3 conversion
A healthy gut microbiome, where part of thyroid hormone conversion occurs
A calm, predictable stress rhythm, which reduces cortisol interference with thyroid hormones
Steady circadian patterns, which support overall endocrine balance
Medication replaces the hormone.
Nutrition and lifestyle support the hormone becoming usable.
These supports do not replace medical treatment for autoimmune thyroid disease, but they strengthen the underlying systems that influence fertility: stable energy, consistent ovulation, calmer cycles and improved early embryo development.
This combination is particularly important when trying to conceive or preparing for IVF, when thyroid hormone demand increases.
Your Next Step
If you suspect that an underactive thyroid or autoimmune thyroid activity is affecting your fertility, you do not have to navigate this alone. Gentle, practical changes can support hormone balance, improve energy, and strengthen the foundations for conception and early pregnancy.
Explore the pathways available and find the level of support that feels right for you.