Thyroid and Fertility
The small gland with a big impact on conception.
Key Points
- Common connection: Thyroid disorders affect 2-4% of women of reproductive age
- Treatable: Most thyroid issues are easily managed with medication
- Optimal TSH: 0.5-2.5 mIU/L is ideal for TTC and early pregnancy
- Test antibodies: Thyroid antibodies matter even with normal TSH
How Your Thyroid Affects Fertility
The thyroid gland—that butterfly-shaped gland at the base of your neck—regulates metabolism throughout your body, including your reproductive system.
Thyroid hormones influence:
- Menstrual cycle regularity
- Ovulation
- Egg quality
- Uterine lining development
- Implantation
- Early pregnancy maintenance
Both hypothyroidism (underactive) and hyperthyroidism (overactive) can impair fertility and increase pregnancy risks.
Hypothyroidism (Underactive Thyroid)
What It Is
The thyroid doesn't produce enough thyroid hormone (T4 and T3). The pituitary gland releases more TSH to try to stimulate production.
Most common cause: Hashimoto's thyroiditis (autoimmune)
Symptoms
- Fatigue
- Weight gain
- Cold intolerance
- Constipation
- Dry skin and hair
- Depression
- Heavy or irregular periods
- Difficulty concentrating
How It Affects Fertility
- Menstrual irregularities: Heavy periods, irregular cycles, anovulation
- Elevated prolactin: Can suppress ovulation
- Luteal phase defects: Short second half of cycle
- Increased miscarriage risk: Especially with untreated or undertreated disease
📊 Subclinical Hypothyroidism
This is when TSH is elevated but T4 is normal. You may have no symptoms. However, even subclinical hypothyroidism may affect fertility and is often treated when TTC.
Hyperthyroidism (Overactive Thyroid)
What It Is
The thyroid produces too much thyroid hormone. TSH is suppressed as the pituitary tries to slow production.
Most common cause: Graves' disease (autoimmune)
Symptoms
- Rapid heartbeat, palpitations
- Unintentional weight loss
- Heat intolerance
- Tremors
- Anxiety, irritability
- Light or absent periods
- Bulging eyes (Graves')
How It Affects Fertility
- Menstrual irregularities: Light, infrequent, or absent periods
- Anovulation: Excess thyroid hormone disrupts ovulation
- Miscarriage risk: Uncontrolled hyperthyroidism increases risk
- Pregnancy complications: Preeclampsia, preterm birth, low birth weight
Thyroid Antibodies Matter
Even with normal TSH levels, the presence of thyroid antibodies (TPO antibodies, thyroglobulin antibodies) may affect fertility.
What Are Thyroid Antibodies?
Antibodies produced by your immune system that attack thyroid tissue. Present in autoimmune thyroid diseases (Hashimoto's, Graves').
Fertility Impact
- Higher rates of infertility even with normal thyroid function
- Increased miscarriage risk (some studies show 2-4x higher)
- May affect egg quality or implantation through immune mechanisms
- Associated with higher rates of IVF failure
⚠️ Test Beyond TSH
If you're struggling to conceive or have had miscarriages, ask for thyroid antibody testing even if your TSH is normal. Many doctors only test TSH unless specifically requested.
Optimal Thyroid Levels for TTC
General population "normal" ranges are not the same as optimal for conception:
| Test | General "Normal" | Optimal for TTC/Pregnancy |
|---|---|---|
| TSH | 0.4-4.5 mIU/L | 0.5-2.5 mIU/L |
| Free T4 | 0.8-1.8 ng/dL | Mid-normal range |
| TPO Antibodies | <35 IU/mL | Negative is ideal |
💡 The 2.5 Rule
Most reproductive endocrinologists recommend keeping TSH under 2.5 mIU/L when trying to conceive and during the first trimester. Some aim for under 2.0. A TSH of 3.5 might be "normal" but isn't optimal for fertility.
Testing & Diagnosis
What Tests to Request
- TSH: Primary screening test
- Free T4: Active thyroid hormone
- Free T3: Sometimes helpful
- TPO antibodies: Detects autoimmune thyroiditis
- Thyroglobulin antibodies: Additional autoimmune marker
When to Test
- Before starting to try to conceive (ideally)
- If you have symptoms of thyroid dysfunction
- If periods are irregular or absent
- After 6-12 months of trying without success
- After any miscarriage
- If you have a family history of thyroid disease
- If you have other autoimmune conditions
Treatment
For Hypothyroidism
Levothyroxine (synthetic T4) is the standard treatment. Brand names include Synthroid, Levoxyl, and Tirosint.
- Take on empty stomach, 30-60 minutes before food
- Avoid taking with calcium, iron, or antacids (interfere with absorption)
- Dose adjustments based on TSH monitoring
- Safe during pregnancy—actually essential if you have hypothyroidism
During pregnancy: Thyroid hormone requirements increase 25-50%. Most women need dose increases starting in the first trimester.
For Hyperthyroidism
Treatment options include:
- Anti-thyroid medications: Methimazole (preferred outside of first trimester), PTU (preferred in first trimester)
- Radioactive iodine: Not safe during pregnancy or breastfeeding; must wait 6 months after treatment before TTC
- Surgery: May be recommended in certain cases
For Thyroid Antibodies with Normal TSH
Treatment is more controversial. Some doctors recommend:
- Low-dose levothyroxine (studies show mixed results)
- Selenium supplementation (may reduce antibody levels)
- Close monitoring during pregnancy
Thyroid and Male Fertility
Thyroid disorders in men also affect fertility:
- Hypothyroidism: Lower sperm count, reduced motility, erectile dysfunction
- Hyperthyroidism: Abnormal sperm morphology, reduced libido
If male partner has symptoms or fertility testing shows issues, thyroid evaluation is warranted.
FAQs
Can I conceive with thyroid disease?
Yes! With proper treatment and monitoring, most women with thyroid disorders conceive and have healthy pregnancies. The key is getting diagnosed and optimizing levels before and during pregnancy.
Will my thyroid medication harm the baby?
Levothyroxine is safe and necessary during pregnancy. Uncontrolled hypothyroidism is far more dangerous to the baby than the medication. Don't stop taking it!
How often should I check my thyroid during pregnancy?
Typically every 4-6 weeks in the first half of pregnancy, then at least once in the third trimester. Your doctor will adjust based on your situation.
Can thyroid problems cause miscarriage?
Untreated or poorly controlled thyroid disease increases miscarriage risk. With proper treatment, this risk is significantly reduced.
The Bottom Line
Thyroid health is often overlooked in fertility discussions, but it's easy to test and treat. If you're struggling to conceive, ask for comprehensive thyroid testing—not just TSH, but also Free T4 and thyroid antibodies.
The good news: thyroid disorders are highly treatable, and optimizing your levels can make a real difference in your fertility. 💚