Many women newly diagnosed with PCOS are surprised when their doctor also orders a thyroid test. The reason is simple: these two conditions share many of the same symptoms, including irregular periods, weight gain, fatigue, and thinning hair. Sometimes a thyroid problem masquerades as PCOS. Sometimes both exist at once. Understanding this overlap matters because treating one condition will not fix the other if the real cause is missed.

Why thyroid and PCOS often overlap

The link between thyroid and PCOS is not coincidence. Studies consistently show women with PCOS have higher rates of thyroid problems than women without it. Subclinical hypothyroidism (slightly elevated TSH but still-normal thyroid hormone) is found in roughly one in five women with PCOS, far higher than in the general population.

More striking is Hashimoto’s thyroiditis, an autoimmune disease in which the immune system attacks the thyroid gland. Several studies show Hashimoto’s is more common in women with PCOS, with some reporting rates around three times higher than in women without PCOS. Anti-TPO antibodies (a marker of thyroid autoimmunity) are also positive more often among women with PCOS.

Why this happens is not fully understood. The leading hypotheses involve oxidative stress, metabolic dysfunction, and shared genetic susceptibility between the two conditions. The practical takeaway: if you have PCOS, your risk of a thyroid problem is somewhat higher, so screening is sensible.

How hypothyroidism can mimic PCOS

This is the part that often confuses people. Hypothyroidism (an underactive thyroid) can produce a picture that looks almost identical to PCOS, so much so that misdiagnosis sometimes happens. The mechanism involves several hormonal pathways:

  • SHBG drops. When thyroid hormone is low, the liver makes less SHBG (the protein that binds testosterone). As a result, more free testosterone circulates even though total testosterone has not risen. This can trigger acne and excess hair (hirsutism), the very symptoms usually linked to PCOS.
  • Prolactin rises. Significant hypothyroidism increases TRH from the brain, which in turn can raise prolactin. High prolactin disrupts ovulation and causes irregular periods.
  • Ovarian appearance changes. Severe hypothyroidism sometimes produces enlarged ovaries with many follicles on ultrasound, looking much like polycystic ovaries. This can mislead if the thyroid is not checked first.

This is exactly why the 2023 international PCOS guideline (Monash/ESHRE/ASRM) lists thyroid function testing (TSH) as part of the workup to rule out other causes before confirming a PCOS diagnosis. PCOS is a diagnosis of exclusion, meaning other conditions that can mimic its symptoms, including thyroid problems and high prolactin, must be ruled out first. You can read more about this diagnostic process in the guide on what PCOS is.

The tests you need and how to read them

The basic thyroid tests usually ordered are:

  • TSH (Thyroid Stimulating Hormone). The most important screening test. High TSH points to an underactive thyroid (hypothyroidism); low TSH points to an overactive thyroid (hyperthyroidism). The normal range is typically around 0.4 to 4.0 mIU/L, but labs differ slightly and doctors interpret it in your context.
  • Free T4. Measures the level of active thyroid hormone. If TSH is high but Free T4 is still normal, that is called subclinical hypothyroidism.
  • Anti-TPO antibodies. If positive, this indicates autoimmune thyroiditis (Hashimoto’s), the most common cause of hypothyroidism in iodine-sufficient areas.

At government clinics, TSH is usually available as a basic test. If results show a problem, the doctor will add Free T4 and antibodies as needed. Do not interpret results yourself without a doctor, because a single TSH value slightly outside the range does not necessarily mean you need immediate treatment. Doctors often repeat the test after a few weeks before deciding.

For Malaysian women, one fact matters: thyroid problems are more common in women than men (roughly five to eight times), and the rate of positive thyroid antibodies among urban Malaysian women is fairly high. So if you have PCOS and persistent fatigue, constipation, or unusually dry skin, mention it so the thyroid is considered too.

When both exist at once

If testing confirms you have both PCOS and hypothyroidism, both conditions can be managed. Thyroid treatment (usually levothyroxine, a replacement thyroid hormone) often improves some of the symptoms you assumed came from PCOS, especially fatigue and cycle regularity. However, levothyroxine is not a cure for PCOS itself. You still need to manage insulin resistance and androgens according to your PCOS plan.

Levothyroxine is a prescription medication that needs periodic TSH monitoring to adjust the dose. Never adjust the dose yourself. For women planning pregnancy, good thyroid control is especially important because untreated hypothyroidism can affect fertility and pregnancy. Discuss with your doctor before conceiving if you have both conditions.

Inositol and the thyroid: what the evidence says

Inositol is well known as a PCOS supplement, but there is also early research on its effect on the thyroid. A study by Nordio and colleagues showed that combined myo-inositol plus selenium can lower TSH and thyroid antibodies (anti-TPO and anti-Tg) in patients with subclinical hypothyroidism and autoimmune thyroiditis, over roughly six months.

Those studies were small and focused on specific groups, so the evidence is still limited to moderate, not strong. Inositol is not a substitute for levothyroxine if you need it. Treat inositol as potential support, not a primary treatment for a thyroid problem.

The dose used in most PCOS studies is myo-inositol and D-chiro-inositol at a 40:1 ratio (around 4,000 mg of myo-inositol), while the thyroid studies combined it with selenium. Do not treat these figures as a prescription; discuss with your doctor, especially if you take thyroid medication. For supplements in Malaysia, check halal status (look for the JAKIM logo) and make sure the product is registered with the National Pharmaceutical Regulatory Agency at npra.gov.my. Excess selenium can be toxic, so do not take high doses without advice. Pregnant or breastfeeding women should discuss first before taking any supplement.

When to see a doctor

See a doctor if you have PCOS and symptoms suggesting a thyroid problem: extreme fatigue that does not improve with rest, unexplained weight gain, constipation, dry skin, feeling unusually cold, or swelling in the neck. If you are planning pregnancy and have never checked your thyroid, ask for a TSH test first.

In Malaysia, start at a government health clinic, which offers basic tests at low cost for citizens, then a referral to a specialist clinic if needed. Bring a list of symptoms and any earlier test results. If you are newly diagnosed with PCOS and unsure of the order of steps, the newly diagnosed guide helps prioritise the first week. For the full picture of other PCOS-related hormones, see the hormones hub.