The short answer: Yes, both an underactive and an overactive thyroid can disturb your cycle and lower your chances of conceiving. The 2026 American Thyroid Association pregnancy guideline says untreated overt hypothyroidism causes menstrual changes in about 23% of cases. Once hypothyroidism is treated, fertility matches that of women without thyroid disease.
How does the thyroid affect your menstrual cycle?
Thyroid hormone acts on the ovaries, the uterus, and the brain signals that control ovulation. A 2010 review in Endocrine Reviews notes that both high and low thyroid levels change sex hormone binding globulin (SHBG) and sex steroid levels. Those shifts can delay ovulation, stop it, or change how much you bleed.
The NIDDK lists "heavy or irregular menstrual periods or fertility problems" as common symptoms of hypothyroidism. Its hyperthyroidism page lists "menstrual cycle and fertility issues" as a complication. Both pages say women who have trouble getting pregnant often get a thyroid test for this reason.
Most women with a thyroid problem still have normal periods. The changes are real, but they are not universal, so a regular cycle does not rule out a thyroid problem.
Which period changes come with an underactive or overactive thyroid?
Underactive thyroid (hypothyroidism). A 1999 study of 171 premenopausal women with hypothyroidism found irregular periods in 23.4%, compared with 8.4% of 214 healthy controls. The most common problems were:
- Oligomenorrhea: infrequent periods, with long gaps between cycles.
- Menorrhagia: heavy or prolonged bleeding.
The same study found that irregular periods were somewhat more common in severe cases than in mild ones, but the difference was not statistically significant. The 2026 ATA guideline also lists amenorrhea (no periods) and light periods. It adds that hypothyroidism accounts for 2% to 3% of all causes of anovulation, which means cycles where no egg is released.
Overactive thyroid (hyperthyroidism). An overactive thyroid tends to make periods lighter or more frequent. The Endocrine Reviews summary links thyrotoxicosis mainly with hypomenorrhea (light bleeding) and polymenorrhea (cycles that come too often).
In a 1994 study of 214 women with thyrotoxicosis, 21.5% had irregular cycles, compared with 8.4% of matched controls. None of the women had stopped having periods altogether. Irregular cycles were more common in smokers and in women with higher T4 levels.
The 2026 ATA guideline also notes that a suppressed TSH was linked to a longer time to pregnancy in a large observational study. It advises treating overt hyperthyroidism before conception.
Does a thyroid problem make it harder to get pregnant?
It depends on how far off your levels are. The evidence is strongest for overt disease and weak for mild changes.
- Overt hypothyroidism: The 2026 ATA guideline calls it "a well-established risk factor for infertility or recurrent miscarriages." It recommends delaying fertility treatment until thyroid levels are back to normal.
- Subclinical hypothyroidism (high TSH, normal T4): The guideline cites a slightly lower chance of conception (an absolute difference of 1.4 to 4.5 percentage points) and a slightly higher miscarriage risk (0.4 to 0.7 percentage points) when TSH is raised but under 10 mU/L.
- Overt hyperthyroidism: The guideline advises treatment before conception.
The 2024 American Society for Reproductive Medicine (ASRM) guideline is more cautious. It says there is insufficient evidence to tell women that subclinical hypothyroidism causes infertility. It also says a TSH between 2.5 and 4.0 mU/L is not linked to a higher miscarriage risk.
Do thyroid antibodies raise the risk of miscarriage?
Thyroid peroxidase antibodies (TPOAb) are the marker of Hashimoto's disease. Many women have them while their thyroid levels are still normal. A 2011 BMJ meta-analysis of cohort studies found these antibodies tripled the odds of miscarriage (odds ratio 3.90). The 2026 ATA guideline puts the absolute increase at about 2 to 8 percentage points.
Early small trials suggested levothyroxine might lower that risk. Larger trials did not confirm it. In the TABLET trial, 952 women with antibodies, normal thyroid levels, and a history of miscarriage or infertility took levothyroxine or a placebo. The live birth rate was 37.4% with levothyroxine and 37.9% with placebo.
Based on TABLET and two other high-quality trials, the 2026 ATA guideline now says levothyroxine "should not be offered" to women with antibodies and normal thyroid levels. It suggests rechecking TSH every 3 to 6 months while you try to conceive. About 7% to 9% of these women develop hypothyroidism within a year. Our article on thyroid antibodies and hypothyroidism risk covers that risk in more detail.
Should you get a TSH test before trying to conceive?
The two main guidelines differ on who should be tested. The 2026 ATA guideline says TSH testing should be done in all non-pregnant women who present with infertility or recurrent miscarriage. The 2024 ASRM guideline does not support universal screening. It advises testing TSH and T4 in women with signs or symptoms of hypothyroidism, "including irregular menstrual cycles."
Both agree that irregular periods are a good reason to check. A practical checklist built from the 2026 ATA guideline:
- Get a TSH test if your cycles are irregular, you have had trouble conceiving, or you have had repeated miscarriages. Ask whether a TPOAb test makes sense too.
- Repeat a mildly high result. The guideline suggests rechecking TSH and free T4 in 4 to 6 weeks, because many single high results return to normal.
- Treat overt hypothyroidism first. Fertility treatment should wait until thyroid levels are normal. Levothyroxine is the recommended medicine. The guideline advises against liothyronine (T3) or desiccated thyroid when planning pregnancy.
- Know your target if you already take levothyroxine. The guideline uses a TSH of 0.5 to 2.5 mU/L before conception and in pregnancy for women with infertility or recurrent miscarriage.
- Plan the timing with Graves' disease. Women on methimazole should switch to propylthiouracil when planning pregnancy. After radioactive iodine, the guideline advises waiting at least 6 months before trying.
- Have a plan for a positive test. The guideline suggests a TSH check about 2 weeks after a positive pregnancy test for women on levothyroxine who had fertility treatment.
Once you are pregnant, the targets change. Our guide to how pregnancy changes thyroid targets explains the trimester ranges and dose increases.
Does treatment bring periods and fertility back to normal?
For overt disease, usually yes. The 2026 ATA guideline states that women with hypothyroidism who are euthyroid (in range) on levothyroxine "have the same fertility outcomes as those without hypothyroidism."
For mild changes, the benefit is less clear. The 2024 ASRM guideline found that treating subclinical hypothyroidism has not been shown to reduce pregnancy loss or raise live birth rates. It recommends against treating it in women trying to conceive. The 2026 ATA guideline is more open: it says low-dose levothyroxine (25 to 75 mcg a day) may be started if subclinical hypothyroidism persists on repeat testing, and should be given when TSH is above 10 mU/L. Your clinician will weigh your TSH level, antibody status, and fertility plans.
Irregular periods have many other causes, such as polycystic ovary syndrome, high prolactin, weight change, and stress. If your thyroid tests are normal, those deserve a look. For help sorting through overlapping symptoms, see symptom.md. If you are in your 40s, our article is it menopause or my thyroid? covers perimenopause.
The bottom line
An underactive thyroid most often causes infrequent or heavy periods. An overactive thyroid more often causes light or frequent periods. Overt thyroid disease can make it harder to conceive and raises miscarriage risk, but treatment usually restores normal fertility. Mild TSH changes and thyroid antibodies have smaller effects, and the 2026 ATA guideline no longer recommends levothyroxine for antibodies alone. If your cycles are irregular or you are struggling to conceive, ask for a TSH test.
Last updated: November 2026. This article is for informational purposes only and does not constitute medical advice. Talk with your clinician or fertility specialist before starting, stopping, or changing any thyroid medication while trying to conceive.