The thyroid is a butterfly-shaped gland in your neck that produces hormones affecting virtually every cell in your body. When thyroid function is off—either too high or too low—it can disrupt your reproductive system at multiple levels.
The good news: thyroid issues are common, easy to test for, and highly treatable. If you're having trouble conceiving or have a history of miscarriage, checking your thyroid should be one of the first steps.
How Thyroid Affects Fertility
Your thyroid hormones (T3 and T4) interact closely with reproductive hormones. When thyroid levels are abnormal, it can impact:
- Ovulation: Thyroid dysfunction can cause irregular or absent ovulation
- Menstrual cycles: Both heavy, irregular periods (hypothyroid) and light, infrequent periods (hyperthyroid)
- Implantation: May affect the uterine lining's ability to support implantation
- Pregnancy maintenance: Increases risk of miscarriage, especially in early pregnancy
- Prolactin: Hypothyroidism can elevate prolactin, further suppressing ovulation
Understanding TSH Levels
TSH (thyroid-stimulating hormone) is the primary screening test for thyroid function. It works inversely to thyroid hormone levels:
- High TSH = hypothyroidism (underactive thyroid)—the pituitary is producing more TSH trying to stimulate a sluggish thyroid
- Low TSH = hyperthyroidism (overactive thyroid)—the pituitary has reduced TSH because there's already too much thyroid hormone
TSH Reference Ranges for Fertility
Many experts recommend tighter control than general population ranges
General labs often use a "normal" TSH range of 0.4-4.5 mIU/L. But for fertility and pregnancy, many reproductive endocrinologists recommend a tighter target of 0.5-2.5 mIU/L. A TSH of 3.5 might be called "normal" on your lab report but could still be suboptimal for conception.
Hypothyroidism (Underactive Thyroid)
Effects on Fertility:
- Irregular or absent ovulation
- Elevated prolactin (which suppresses ovulation)
- Luteal phase defects
- Increased miscarriage risk (2-4x higher)
- Higher risk of pregnancy complications
Treatment:
Levothyroxine (synthetic T4) is the standard treatment. It's safe during pregnancy and should be continued (often at an increased dose). Most women respond well to treatment, with improved ovulation and pregnancy rates once TSH is optimized.
Subclinical hypothyroidism—where TSH is mildly elevated (typically 4.0-10) but T4 is still normal—is debated. However, most fertility specialists recommend treating it when TTC, as even mild thyroid insufficiency may affect conception and increase miscarriage risk.
Hyperthyroidism (Overactive Thyroid)
Effects on Fertility:
- Irregular menstrual cycles (often light or infrequent)
- Anovulation
- Increased miscarriage risk
- Pregnancy complications (preeclampsia, preterm birth, heart problems)
Treatment:
Depends on the cause. Options include anti-thyroid medications (methimazole, propylthiouracil), radioactive iodine, or surgery. Radioactive iodine requires waiting 6+ months before TTC. Control hyperthyroidism before conceiving—pregnancy can worsen the condition.
Hashimoto's Thyroiditis
Hashimoto's is an autoimmune condition where your immune system attacks your thyroid, eventually causing hypothyroidism. It's the most common cause of hypothyroidism in developed countries and deserves special attention for fertility:
- Thyroid antibodies (TPO antibodies, thyroglobulin antibodies) are present even when TSH is still "normal"
- Women with positive antibodies have 2-3x higher miscarriage rates, even with normal TSH
- Treatment debate: Some evidence suggests levothyroxine may benefit women with positive antibodies even with normal TSH, though this is controversial
- Selenium supplementation may help reduce antibody levels (see below)
If you have Hashimoto's and are TTC, work closely with your doctor to keep TSH at the lower end of normal (under 2.5) and monitor closely during pregnancy.
Symptoms to Watch For
Hypothyroid Symptoms
- Fatigue, sluggishness
- Weight gain
- Feeling cold
- Dry skin, brittle nails
- Hair loss or thinning
- Constipation
- Depression
- Brain fog, poor memory
- Heavy or prolonged periods
- Muscle aches
Hyperthyroid Symptoms
- Anxiety, nervousness
- Weight loss
- Feeling hot, sweating
- Rapid heartbeat
- Tremors in hands
- Difficulty sleeping
- Diarrhea
- Light or infrequent periods
- Eye problems (Graves')
- Muscle weakness
Many women have subclinical thyroid issues with few or no symptoms. This is why testing is important—you can't always feel that something is off.
Getting Tested
Thyroid testing should be part of any basic fertility workup. The tests include:
TSH: The primary screening test. If abnormal, further testing is needed.
Free T4: The active thyroid hormone. Helps differentiate overt vs. subclinical thyroid disease.
TPO antibodies: Tests for Hashimoto's thyroiditis. Important because antibodies are associated with miscarriage risk even with normal TSH.
Free T3: Sometimes checked, but less important for fertility than TSH and T4.
Get your thyroid checked before TTC if you have symptoms, family history of thyroid disease, other autoimmune conditions, or a history of miscarriage. Once pregnant, thyroid needs increase—retest early in pregnancy and adjust medication as needed.
Supporting Thyroid Health
While medication is essential for treating thyroid disease, certain nutrients support thyroid function:
Vitamin D: Deficiency is associated with autoimmune thyroid disease. Test your levels and supplement if low.
Iron: Iron is needed for thyroid hormone production. Iron deficiency is common in women with heavy periods (which hypothyroidism can cause).
Zinc: Supports thyroid hormone conversion. Most people get enough from diet.
Thyroid During Pregnancy
Thyroid hormone requirements increase by 25-50% during pregnancy. If you're on thyroid medication:
- Don't stop your medication—it's safe during pregnancy and essential for baby's brain development
- Test immediately when you get a positive pregnancy test
- Your dose will likely need to increase—some doctors recommend increasing levothyroxine by 25-30% as soon as pregnancy is confirmed
- Monitor regularly—TSH should be checked every 4-6 weeks in the first half of pregnancy
- Target TSH in pregnancy: First trimester: <2.5 mIU/L; Second/third trimester: <3.0 mIU/L
Not Sure Where to Start?
Our quiz can help you understand your fertility situation and point you to the right resources.
Take the Fertility Quiz →The Bottom Line
Thyroid problems are one of the most treatable causes of fertility issues:
- Get tested—a simple blood test can identify thyroid problems
- Target TSH of 0.5-2.5 when trying to conceive (tighter than general population ranges)
- Treat subclinical hypothyroidism when TTC—don't wait for overt disease
- Check thyroid antibodies—even with normal TSH, antibodies increase miscarriage risk
- Control hyperthyroidism before TTC—it's riskier in pregnancy
- Monitor closely in pregnancy—hormone requirements increase significantly
If you have a thyroid condition and are having trouble conceiving, optimizing your thyroid is one of the simplest and most effective interventions available.