Differential Diagnosis of PCOS

There are medical conditions that cannot be diagnosed with a blood test, biopsy, culture, or any other form of diagnostic testing. Polycystic ovary syndrome (PCOS) is one such condition. Because the symptoms of PCOS can mimic those of other conditions, doctors need to methodically exclude all other causes before a diagnosis can be made.

Doctor and patient talking in exam room
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This process is called a differential diagnosis. It is only by narrowing the list of suspects, one by one, that doctors can come to a definitive conclusion and start treatment.

In the course of establishing a differential diagnosis for PCOS, some of the more common investigations would include thyroid disease, hyperprolactinemia, congenital adrenal hyperplasia, and Cushing's syndrome. Depending on the health and history of the individual, other causes may also be explored.

Thyroid Disease

The thyroid gland is a small organ located at the front of the throat that regulates much of the body’s metabolism. It produces two hormones, known as T3 and T4, which are vital to regulating many key bodily functions including respiration, heart rate, body weight, muscle strength, and menstrual cycles.

When there is either too little thyroid hormone (hypothyroidism) or too much (hyperthyroidism), these functions can be thrown into chaos, leading to symptoms that are strikingly similar to PCOS. These can include abnormal menstrual cycles, unexplained changes in weight, fatigue, temperature intolerance, and, in the case of hypothyroidism, infertility.

Thyroid disease can be diagnosed by performing blood tests to evaluate the T3 and T4 levels. Further tests would be performed to pinpoint the underlying cause.


Prolactin is a hormone produced by the pituitary gland which acts on the mammary glands to promote lactation. Hyperprolactinemia is a condition where too much prolactin is produced, leading to irregular menstruation and lactation (galactorrhea). PCOS can similarly trigger increases in prolactin levels.

One of the more common causes of hyperprolactinemia is a pituitary tumor called a prolactinoma. A prolactinoma can either be large or small and is most often benign (noncancerous). Hyperprolactinemia also requires a differential diagnosis to exclude thyroid disease as a cause. A magnetic resonance imaging (MRI) test can be used to confirm the presence of a tumor.

Congenital Adrenal Hyperplasia

Congenital adrenal hyperplasia (CAH) is a genetic disorder that causes the adrenal glands to produce too little cortisol and aldosterone. Cortisol is the body’s main stress hormone, while aldosterone helps regulate the level of sodium and other electrolytes in the body. At the same time, CAH can trigger the excessive production of androgens, the hormones associated with male characteristics.

These imbalances can lead women to experience irregular periods, excessive hair growth (hirsutism), and the failure to menstruate (amenorrhea). Unlike PCOS, CAH can be diagnosed with a genetic test.

Cushing's Syndrome

Cushing's syndrome is a disease that causes the overproduction of adrenal hormones. It is typically caused by a benign tumor called a pituitary adenoma which alters the activity of the adrenal gland and triggers the excess production of cortisol and androgens.

Cushing's syndrome is characterized by symptoms that are strikingly similar to PCOS, including weight gain, hirsutism, facial puffiness, increased urination, and changes in skin texture.

As with PCOS, there is no single test to confirm a Cushing's diagnosis. Typically, urine and saliva tests are performed to measure and evaluate the pattern of cortisol production consistent with Cushing's syndrome.

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Additional Reading
  • William, T.; Mortada, R.; and Porter, S. "Diagnosis and Treatment of Polycystic Ovary Syndrome." Amer Fam Phys. 2016; 94(2):106-13. PMID: 27419327.