Thyroid and Fertility: The Underdiagnosed Connection
Thyroid dysfunction is one of the most underdiagnosed and easiest-to-treat causes of fertility problems. A simple blood test and an inexpensive daily pill can make all the difference.
Subclinical hypothyroidism (TSH 2.5–4.0 mIU/L) is associated with longer time to conception and higher miscarriage risk. The target TSH for fertility is ≤2.5 mIU/L. Treatment with levothyroxine is simple and effective.
How Thyroid Affects Fertility
The thyroid gland regulates metabolism, energy, and hormonal balance throughout the body. When it underperforms (hypothyroidism), it can disrupt ovulation, reduce progesterone production, interfere with implantation, and increase miscarriage risk.
The tricky part: you can have subclinical hypothyroidism — TSH levels that are “normal” by general standards (0.5–4.5 mIU/L) but suboptimal for fertility (above 2.5 mIU/L) — and experience reduced fecundability with no other symptoms.
The Numbers That Matter
| TSH Level | General Classification | Fertility Implication |
|---|---|---|
| <0.5 mIU/L | Possibly hyperthyroid | May disrupt ovulation; evaluate further |
| 0.5–2.5 mIU/L | Optimal for fertility | No thyroid-related fertility concern |
| 2.5–4.0 mIU/L | “Normal” but suboptimal | May increase time to conception and miscarriage risk |
| >4.0 mIU/L | Hypothyroid | Should be treated before TTC |
Thyroid Autoimmunity
Even with normal TSH, elevated thyroid peroxidase (TPO) antibodies indicate thyroid autoimmunity (Hashimoto's thyroiditis) and are independently associated with higher miscarriage rates. About 5–10% of reproductive-age women have elevated TPO antibodies. Some REs treat with levothyroxine even when TSH is normal if TPO antibodies are significantly elevated.
Treatment
Levothyroxine (synthetic thyroid hormone) is the standard treatment. It's inexpensive (often under $10/month), well-tolerated, and taken as a daily pill on an empty stomach. Dosing is adjusted based on repeat TSH testing every 4–6 weeks until optimal levels are achieved.
During pregnancy, levothyroxine requirements typically increase by 30–50%. Your provider will monitor TSH throughout the first trimester and adjust accordingly.
When you get your fertility bloodwork, make sure TSH is included — and ask for TPO antibodies as well. If your TSH is above 2.5, discuss treatment with your provider specifically in the context of fertility, not just general thyroid guidelines.