Polycystic Ovary Syndrome (PCOS): Although the presence of cysts is often associated with PCOS, their absence does not rule out the diagnosis. PCOS is characterized by a combination of hyperandrogenism (hirsutism), ovulatory dysfunction (irregular cycles), and polycystic ovaries on ultrasound, but not all criteria must be present for a diagnosis. The clinical presentation of irregular cycles and hirsutism is highly suggestive of PCOS, even in the absence of ovarian cysts.

Idiopathic Hirsutism: This condition presents with excessive hair growth without other signs of hyperandrogenism or ovulatory dysfunction. However, it can sometimes be difficult to distinguish from PCOS, especially if the patient has mild symptoms.
Thyroid Dysfunction: Both hypothyroidism and hyperthyroidism can cause menstrual irregularities. Hypothyroidism, in particular, can also lead to increased androgen levels, potentially causing hirsutism.
Hyperprolactinemia: Elevated prolactin levels can disrupt ovulation, leading to irregular menstrual cycles. It may also contribute to hirsutism, although this is less common.

Congenital Adrenal Hyperplasia (CAH): This is a group of inherited disorders that affect the adrenal glands, leading to an overproduction of androgens. It can cause hirsutism and menstrual irregularities. Although rare, missing this diagnosis can have significant consequences, including an increased risk of adrenal crisis.
Androgen-Secreting Tumors: Although rare, tumors of the ovaries or adrenal glands can produce excess androgens, leading to hirsutism and potentially disrupting menstrual cycles.
Cushing's Syndrome: This condition, caused by excess cortisol, can lead to hirsutism, menstrual irregularities, and other systemic symptoms. It is crucial to diagnose and treat Cushing's Syndrome due to its potential for serious complications.

Acromegaly: Caused by excess growth hormone, usually due to a pituitary tumor, acromegaly can lead to menstrual irregularities and, less commonly, hirsutism.
Primary Ovarian Insufficiency: This condition, where the ovaries stop functioning and hormone levels decrease, can cause irregular periods but is less commonly associated with hirsutism.
Gonadal Dysgenesis: Conditions like Turner syndrome can lead to ovarian dysfunction and irregular cycles but are typically identified earlier in life due to other characteristic features.