Thyroid disorders and fertility
An underactive thyroid (hypothyroidism) can disrupt ovulation and periods, and is a common, treatable cause of difficulty conceiving. It is checked with a simple TSH blood test. Clearly high TSH is treated with a daily tablet, which usually restores normal cycles; the evidence for treating borderline (subclinical) results is less settled and depends on where your number falls.
The thyroid produces hormones that regulate metabolism, and it also has a direct effect on the menstrual cycle. An underactive thyroid (hypothyroidism) can disrupt or stop ovulation, cause irregular periods, and raise the risk of miscarriage if untreated, which makes it one of the more straightforward, treatable causes of difficulty conceiving worth ruling out early.
How it is checked
The main test is a TSH blood test. A clearly high TSH, generally above about 4.5 to 5.0 mIU/L, points to an underactive thyroid. Many clinics also check thyroid antibodies, since autoimmune thyroid disease is common and can matter for pregnancy even when hormone levels look normal.
Clear versus borderline results
| What it usually means | |
|---|---|
| TSH above about 4.0 to 4.5 | Fair evidence linking this range to miscarriage risk; treatment is generally recommended and tends to improve pregnancy rates |
| TSH roughly 2.5 to 4.0 (subclinical) | A borderline zone; evidence that treatment changes fertility outcomes is inconsistent, and practice varies by clinic |
| TSH within the standard reference range | Thyroid is not a likely cause on its own; other causes should be explored |
Treatment
An underactive thyroid is treated with a daily tablet (levothyroxine) that replaces the missing hormone, adjusted over time using repeat TSH tests. For most people this is inexpensive, well tolerated, and often restores regular ovulation on its own within a few months. If irregular ovulation continues despite a corrected TSH, a doctor may add ovulation-support treatment on top.
Questions worth asking
If you have irregular periods or unexplained difficulty conceiving, it is worth asking whether a TSH test has been done, what your specific number was, and if it is borderline, why your doctor is or is not recommending treatment. Our when to seek help guide covers where this fits into a first assessment.
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