What is TSH (Thyroid-Stimulating Hormone)? Levels & fertility

Nothing in this article is or should be construed as medical advice; for any medical questions, consult your healthcare professional.

Thyroid-stimulating hormone (TSH) plays a behind-the-scenes role in your menstrual cycle, fertility and pregnancy. When TSH levels are out of balance, it can interfere with ovulation, affect cycle regularity¹ and make it harder to conceive. Understanding how TSH works, what’s considered normal and when thyroid levels may need attention can help you know when to talk to your healthcare professional. Let’s dive in.

What is TSH?

TSH stands for thyroid-stimulating hormone.¹ It’s produced by the pituitary gland and acts as a messenger, telling your thyroid when to release two important thyroid hormones: T4 (thyroxine) and T3 (triiodothyronine).^1,2 These hormones regulate metabolism² and support many essential body functions, including menstrual cycles, ovulation and fertility.

Because thyroid hormones work closely with reproductive hormones, even small imbalances can disrupt the signals that help keep your cycle on track. When thyroid hormone levels are too high or too low, it can disrupt the hormonal signals that help regulate your menstrual cycle and ovulation.

What does TSH do?

TSH works through a hormone pathway:

• The pituitary gland releases TSH.
• TSH signals the thyroid gland to produce hormones.
• The thyroid produces T4 (thyroxine).
• T4 is converted into T3 (triiodothyronine), the active hormone used by the body.
• These hormones regulate metabolism and influence reproductive hormones.

The thyroid hormones T4 and T3 control your metabolic rate, which is the speed at which your body converts food into energy.¹ These hormones also affect your heart, muscle control and strength, digestion, weight, body temperature, bone maintenance and mood.^1,3

According to the American Thyroid Association (ATA), during pregnancy, your baby depends on you for the production of the thyroid hormone during the first 18 to 20 weeks of pregnancy.⁴ After this time, your baby begins making their own thyroid hormone but continues to rely on you for iodine, which is essential for making the thyroid hormone.⁴ ATA recommends that women who are trying to conceive (TTC), pregnant or breastfeeding take a daily supplement containing 150 mcg of iodine.⁵ Iodine can also be found in fish, milk, cheese, yogurt, enriched or fortified cereal and bread, and iodized salt.⁵

What are normal TSH levels in females?

It’s important to note that normal TSH levels vary for each individual person. Talk to your healthcare professional to determine your normal range; they’ll determine if your TSH levels are normal based on several factors including your age.¹ TSH levels fluctuate a bit during pregnancy, rising slightly in the second and third trimesters.¹

Can thyroid levels affect reproductive health? 

Yes. Thyroid hormones play an important role in keeping your reproductive system working smoothly. When thyroid levels are out of balance — either too high or too low — they can disrupt the hormonal signals that regulate ovulation and menstruation.⁶ Thyroid disorders have also been linked to the early onset of menopause.⁶

Thyroid imbalance has been linked to changes in cycle length, irregular or absent periods, irregular ovulation and anovulation (when ovulation stops altogether), all of which can make it more difficult to conceive.⁶ In fact, menstrual irregularities are more common in people with thyroid disorders than in those with normal thyroid function.⁷

Thyroid imbalances can also affect pregnancy outcomes. Research has found that uncontrolled thyroid conditions have been associated with an increased risk of early pregnancy loss and preterm birth.⁸ During pregnancy, having too little thyroid hormone may increase the risk of complications like early pregnancy loss and preterm delivery.⁶ Having too much of the thyroid hormone can contribute to symptoms such as more severe morning sickness.⁶

If you have any concerns regarding your thyroid levels, talk to your doctor.

Does your TSH level ever change?

Certain disorders can cause your thyroid gland to make too much or too little T4. (If you have any concerns that you may have any of these, reach out to your doctor.)³

• Hypothyroidism¹: If you don’t have enough T4, your body functions begin to slow down.2,3 Your pituitary gland then begins to make larger amounts of TSH to prompt your thyroid to work harder.²
• Hyperthyroidism¹: If you have too much T4, your body functions speed up.^2,3 Your pituitary gland then makes little or no TSH.²

What are the symptoms of high and low TSH levels?

The symptoms of high and low TSH (hypothyroidism and hyperthyroidism, respectively) can affect your menstrual cycles and mental and physical health. This chart shows some of the differences between the two.

What causes high TSH levels?

High TSH levels could mean you’re experiencing hypothyroidism, in which your thyroid isn’t making enough thyroid hormone.¹ When thyroid hormone levels are low, the pituitary gland releases more TSH to signal the thyroid to produce more T4 hormone.¹

Common causes of high TSH levels include:

• Hashimoto disease, an autoimmune condition that causes thyroid inflammation (thyroiditis)²
• Autoimmune disorders that affect thyroid function¹¹
• Radiation treatment to the neck or thyroid area¹¹
• Surgical removal of part of the thyroid gland¹¹
• Congenital hypothyroidism, meaning the thyroid doesn’t function normally from birth¹¹
• Certain medications that affect thyroid hormone production¹¹
• Damage to the pituitary gland¹¹
• Too much or too little iodine in the diet^2,11

What causes low TSH levels?

Low TSH levels could mean your thyroid is making too much thyroid hormone, which is called hyperthyroidism.1 When thyroid hormone levels are high, the pituitary gland reduces TSH production.¹

Possible low TSH causes include:

• Hyperthyroidism, where the thyroid produces excess thyroid hormone¹⁰
• Autoimmune conditions affecting the thyroid¹⁰
• Viral infections that cause inflammation of the thyroid gland¹⁰

What does high TSH mean for fertility? 

Many people with thyroid disease (including those with hypothyroidism) can have healthy pregnancies with treatment.2,9 However, hypothyroidism can cause irregular periods and irregular ovulation, which can make TTC more difficult. The condition can also increase the risk of early pregnancy loss.8 Because untreated thyroid disease can cause problems for both mom and baby, it’s important to treat hypothyroidism while TTC and throughout pregnancy.2,9 If you suspect an issue with your thyroid, see your healthcare professional.

What does low TSH mean for fertility?

With treatment, having a healthy pregnancy is often possible for people with overactive thyroids.9 As with hypothyroidism, early pregnancy loss has been linked to hyperthyroidism.8 Hyperthyroidism also can cause irregular periods,9 which can affect your chances of getting pregnant. If you have any concerns about hyperthyroidism, reach out to your doctor, as they will be in the best position to advise you.

TSH and fertility: key takeaways
Irregular (high or low) levels of TSH can cause irregular periods and ovulation, which can:

• Make TTC more difficult
• Increase the risk of early pregnancy loss9

How is TSH tested? 

Your healthcare professional will test your TSH levels with a blood test. Like other blood tests you may have had in the past, they’ll take a sample of your blood from a vein in your arm and send it to a lab for testing.¹

How can you lower TSH levels?

If your TSH levels need to decrease, it’s likely because your thyroid gland isn’t making enough thyroid hormone (hypothyroidism). In many cases, hypothyroidism is treated with medication that will gradually increase your TSH levels over time.² Have a discussion with your healthcare professional to understand your options for addressing the issue.

How do you increase TSH levels?

If your TSH levels need to increase, it’s likely because you are experiencing hyperthyroidism and your thyroid gland is making too much thyroid hormone. How you manage this is individual and will be determined by you and your healthcare professional. They may recommend treatment with anti-thyroid medication or surgery.²

If you’re trying to conceive or notice changes in your cycle, talking with your healthcare professional about thyroid health may be helpful. A simple blood test can measure TSH levels and help identify whether your thyroid is working as it should. With the right care and treatment, many people with thyroid disorders are able to regulate their cycles and have healthy pregnancies.