Thyroid Disorders & Fertility

Thyroid disorders are conditions where the thyroid, a small butterfly-shaped gland in your neck, makes too little or too much thyroid hormone. They matter a lot for fertility, and they're common in India: an eight-city study found hypothyroidism in 10.95% of adults, rising to 15.86% in women (Unnikrishnan, 2013).

In hypothyroidism, the gland is underactive and your body runs slow. In hyperthyroidism, it's overactive and runs fast. Both can disturb your menstrual cycle and your ability to conceive. The good news is that thyroid problems are easy to test for and, in most cases, simple to treat.

The most common cause of hypothyroidism is autoimmune thyroiditis (Hashimoto's), where the immune system attacks the thyroid. In the same Indian study, 21.85% of people tested positive for anti-TPO antibodies, the marker of this autoimmunity (Unnikrishnan, 2013).

Iodine matters too. Both too little and too much iodine can upset the gland, which is why intake during pregnancy is carefully guided. Hyperthyroidism is most often caused by Graves' disease, another autoimmune condition. Risk factors include being female, a family history of thyroid or autoimmune disease, a personal history of other autoimmune conditions, and previous neck radiation.

Symptoms depend on whether the thyroid is under or overactive, and many overlap with everyday tiredness, so they're easy to miss. An underactive thyroid tends to slow you down. An overactive one speeds you up.

With hypothyroidism you may notice fatigue, weight gain, feeling cold, dry skin, constipation, heavy or irregular periods, and low mood. With hyperthyroidism you may notice weight loss despite eating well, a racing or pounding heart, heat intolerance, sweating, anxiety, tremor, and lighter or absent periods. Subclinical disease often causes few or no symptoms, which is why a blood test is the only reliable way to know.

Diagnosis starts with a simple blood test for TSH (thyroid stimulating hormone), the most sensitive screen for thyroid function. A high TSH suggests an underactive thyroid; a low TSH suggests an overactive one.

If TSH is abnormal, your doctor checks free T4 to confirm the diagnosis and grade its severity. When TSH is mildly raised but free T4 is still normal, that's called subclinical hypothyroidism, found in about 8% of Indian adults (Unnikrishnan, 2013). An anti-TPO antibody test then shows whether autoimmunity is the cause, which helps predict whether the thyroid will worsen over time and in pregnancy.

Standard Diagnostic Tests in India

• TSH (thyroid stimulating hormone) — The first and most sensitive blood test for thyroid function and the standard fertility screen.
• Free T4 — Measures circulating thyroid hormone to confirm and grade an under or overactive thyroid.
• Anti-TPO antibodies — Detects autoimmune thyroid disease (Hashimoto’s), the most common cause of hypothyroidism.
• TSH receptor antibodies (TRAb) — Used when hyperthyroidism is suspected to confirm Graves’ disease, especially before or during pregnancy.
• Thyroid ultrasound — Assesses gland size and nodules when the examination or blood tests suggest a structural problem.

Thyroid hormones help regulate the menstrual cycle, ovulation, and the early support of a pregnancy, so an imbalance can affect every step. Hypothyroidism can stop ovulation, lengthen or disturb periods, and raise prolactin, all of which make conceiving harder.

Thyroid autoimmunity carries its own risk even when hormone levels look normal. A large meta-analysis found that women with thyroid antibodies had nearly four times the odds of miscarriage (odds ratio 3.90) and roughly double the odds of preterm birth (Thangaratinam, 2011). Hyperthyroidism can reduce menstrual flow, cause cycles to stop, and increases the risk of miscarriage and other pregnancy complications (Endotext, 2024).

• Disrupted ovulation: Both under and overactive thyroid states can stop or disturb ovulation, making it harder to conceive each month.
• Higher miscarriage risk: Thyroid antibodies are linked to nearly four times the odds of miscarriage even when hormone levels are normal (Thangaratinam, 2011).
• Implantation and early pregnancy: Adequate thyroid hormone supports implantation and the first weeks of pregnancy before the baby’s own thyroid develops.
• Raised prolactin: Hypothyroidism can push up prolactin, which further suppresses ovulation and lengthens cycles.
• Lower IVF success if untreated: Treating subclinical hypothyroidism with levothyroxine more than doubled the IVF delivery rate in trials (Velkeniers, 2013).

Treatment is usually straightforward and highly effective. Hypothyroidism is treated with levothyroxine, a once-daily tablet that replaces the missing hormone. The dose is adjusted using repeat TSH tests until your levels sit in the target range.

Hyperthyroidism is managed with anti-thyroid medicines, and sometimes radioactive iodine or surgery, though treatment choices change when pregnancy is planned. Iodine intake is balanced carefully: WHO advises about 250 micrograms of iodine a day during pregnancy, and the American Thyroid Association recommends a 150 microgram daily iodine supplement when planning pregnancy and while pregnant (WHO ELENA; ATA, 2017). Don't take high-dose iodine on your own, as too much can harm the thyroid.

Before IVF, most clinics check and optimise your thyroid, because getting it right improves results. For women undergoing assisted reproduction, the American Thyroid Association recommends treating to keep TSH below 2.5 mIU/L (ATA, 2017).

Treatment can change outcomes. In women with subclinical hypothyroidism having IVF or ICSI, levothyroxine more than doubled the delivery rate (relative risk 2.76) and roughly halved miscarriage (relative risk 0.45) across randomised trials (Velkeniers, 2013). Ovarian stimulation can also raise thyroid demand, so your TSH may be rechecked during the cycle and your levothyroxine dose increased once you're pregnant.