TSH

What it is

TSH (thyroid-stimulating hormone, also called thyrotropin) is produced by the pituitary gland to stimulate the thyroid to produce its hormones (T3 and T4). It acts as a regulator: when the thyroid is underactive (hypothyroidism), the pituitary raises TSH to drive more production. Conversely, when the thyroid is overactive, TSH falls.

TSH is the first-line marker of thyroid function in clinical practice. It can detect both hypothyroidism (high TSH) and hyperthyroidism (low TSH).

Why it matters in PCOS

The link between PCOS and thyroid disease is well established. People with PCOS have a prevalence of autoimmune Hashimoto's thyroiditis that is 5 to 10 times higher than in the general population. Hashimoto's is the most common cause of hypothyroidism in women of reproductive age.

Hypothyroidism can closely mimic many PCOS symptoms: menstrual irregularities, chronic fatigue, weight gain, mood changes, and difficulty conceiving. Missing it can lead to an incomplete PCOS diagnosis and inadequate treatment.

The 2023 ESHRE international PCOS guideline explicitly recommends measuring TSH and anti-TPO antibodies (anti-thyroid peroxidase) in the initial workup of anyone suspected of having PCOS, even in the absence of obvious thyroid symptoms.

Normal values

• Normal TSH: 0.4–4.0 mIU/L
• Subclinical hypothyroidism: TSH 4.0–10 mIU/L with normal T4
• Overt hypothyroidism: TSH > 10 mIU/L with low T4
• Hyperthyroidism: TSH < 0.1 mIU/L
• Pregnancy (first trimester): lower threshold reduced to 0.1–2.5 mIU/L per guidelines

Positive anti-TPO with a normal TSH signals a potentially progressive autoimmune thyroiditis: annual monitoring is recommended.

When is it measured?

TSH (and anti-TPO) is included in the initial PCOS panel per the 2023 ESHRE guideline. It can be measured at any time, with no cycle-day or fasting requirement. In people with treated hypothyroidism, TSH is monitored regularly (every 6–12 months).

Key takeaways

• Hashimoto's thyroiditis is 5–10× more prevalent in PCOS
• Hypothyroidism can mimic PCOS — always screen in initial workup
• Normal TSH: 0.4–4.0 mIU/L
• ESHRE 2023: TSH + anti-TPO recommended in all initial PCOS panels
• Positive anti-TPO with normal TSH: monitor annually