The thyroid gland may be small, but its effects on reproductive health are significant. Thyroid hormones regulate metabolism, but they also directly interact with the hormones that control your menstrual cycle and fertility. Thyroid disorders — particularly hypothyroidism — are among the most common and most underdiagnosed causes of irregular periods and difficulty conceiving.
How the Thyroid Affects Reproductive Health
Thyroid hormones (T3 and T4) interact with the hypothalamic-pituitary-ovarian (HPO) axis — the hormonal cascade that controls your cycle. When thyroid function is disrupted:
- The hypothalamus and pituitary gland can't regulate FSH and LH properly, disrupting ovulation
- Sex hormone binding globulin (SHBG) levels change, affecting how oestrogen and testosterone circulate
- Prolactin can rise (hypothyroidism can cause elevated prolactin), further suppressing ovulation
- Progesterone production in the luteal phase may be impaired, affecting implantation
Symptoms of Hypothyroidism in Women
Hypothyroidism (underactive thyroid) is significantly more common in women and tends to develop gradually, which means it often goes unrecognised for years. Symptoms include:
Fatigue and sluggishness
Profound, persistent tiredness that doesn't improve with sleep — the most common hypothyroidism symptom.
Unexplained weight gain
Difficulty losing weight or unexplained weight gain despite no dietary changes, caused by slowed metabolism.
Heavy or irregular periods
Hypothyroidism commonly causes heavy, prolonged, or irregular periods — sometimes very frequent (menorrhagia) or absent cycles.
Feeling cold
Always feeling cold or having cold hands and feet — the body's metabolism is running too slowly to generate normal heat.
Dry skin and hair loss
Dry, flaky skin, brittle nails, and diffuse hair thinning are common signs that thyroid levels are insufficient.
Brain fog and depression
Difficulty concentrating, memory problems, and depression are frequently associated with hypothyroidism and often improve with treatment.
Thyroid and Trying to Conceive
Even subclinical hypothyroidism (where TSH is elevated but T4 is still normal) can affect fertility and increase miscarriage risk. The American Thyroid Association recommends treating TSH levels above 2.5 mIU/L in women trying to conceive.
Hyperthyroidism (overactive thyroid) also affects fertility, typically causing irregular or absent periods, and needs treatment before trying to conceive.
A standard thyroid test usually checks TSH only. If you're struggling to conceive or have irregular cycles, ask specifically for TSH, Free T4, Free T3, and thyroid antibodies (TPO-Ab and TgAb). Antibody presence alone can affect fertility even when TSH is normal.
Thyroid Testing — What to Ask For
- TSH (thyroid stimulating hormone) — the standard screening test; in the context of fertility, optimal range is 0.5–2.5 mIU/L
- Free T4 (thyroxine) — the main thyroid hormone produced by the gland
- Free T3 (triiodothyronine) — the active form; some people have normal TSH/T4 but poor T4-to-T3 conversion
- TPO antibodies and TgAb — detect Hashimoto's thyroiditis, an autoimmune condition that causes gradual thyroid damage
Treatment
Hypothyroidism is treated with levothyroxine — a synthetic T4 hormone taken as a daily tablet. Most people feel significantly better within 4–8 weeks of reaching the right dose. Treatment is highly effective at normalising cycles and improving fertility outcomes.
If you have Hashimoto's (autoimmune hypothyroidism), treatment doesn't cure the autoimmunity but manages the hormone deficiency. Some people with Hashimoto's also benefit from a gluten-free diet (discuss with your doctor — this is somewhat controversial but some evidence supports it for antibody reduction).
Thyroid in Pregnancy
Thyroid requirements increase by 30–50% during pregnancy. If you're on levothyroxine, your dose will likely need to increase as soon as you know you're pregnant — your doctor should monitor and adjust your levels every 4–8 weeks during the first trimester. Untreated hypothyroidism in pregnancy is associated with miscarriage, preterm birth, and developmental effects on the baby.
Track Every Step of Your TTC Journey
Two Pink Lines is a printable planner designed for women trying to conceive — track cycles, ovulation signs, symptoms, appointments, and emotional milestones in one beautiful tracker.