The most likely diagnosis is alopecia areata, specifically patchy alopecia areata.

It classically causes a sudden, sharply defined round or oval patch of complete hair loss with smooth, normal-appearing scalp skin and no scale, redness, or pigment change. It is autoimmune and can occur in otherwise healthy young adults.

A dermatologist can confirm it clinically or with dermoscopy, often finding short “exclamation-mark” hairs at the edge. Other causes to rule out include fungal scalp infection, traction-related loss, and scarring alopecia, but those more often have scaling, inflammation, broken hairs, or loss of follicular openings. The AAD description of alopecia areata matches this presentation closely.

For a single, limited patch of alopecia areata, treatment is usually:

For rapidly spreading, extensive, or recurrent disease, dermatologists may consider short-term oral corticosteroids, contact immunotherapy, or systemic JAK-inhibitor treatment. These are not typically needed for one small patch and require specialist monitoring.

Arrange a dermatology assessment before treatment to confirm the diagnosis. A fungal scalp infection can occasionally mimic patchy hair loss, but it usually has scale, inflammation, broken hairs, or itch. Current guidance supports potent topical steroids and, for limited adult disease, intralesional triamcinolone as standard options, as summarized in the BAD guideline overview.

Yes, spontaneous regrowth is quite possible if this is a single small patch of alopecia areata.

A patch with a 2 cm radius is localized disease. Being an adult with no family history and no reported nail changes, widespread hair loss, or autoimmune illness are generally reassuring features. In people with patchy disease of less than one year, at least half have spontaneous regrowth within a year, though the exact timing cannot be predicted. New hairs may first look fine, white, or lighter than usual before becoming normal in texture and color. DermNet's prognosis summary also lists family history, extensive disease, nail dystrophy, long duration, and an ophiasis pattern as less favorable features.

However, because the patch is on the back of the head, it is worth having a dermatologist confirm the pattern. A band-like loss extending around the back and sides of the scalp is called ophiasis and has a less favorable prognosis. A single round 2 cm patch at the occiput is not necessarily ophiasis.

Seek review sooner if it enlarges, new patches appear, there is eyebrow/eyelash loss, nail pitting, scaling/itch/pain, or there is no early regrowth over about 3 to 6 months. Treatment can promote faster regrowth but is not always necessary for one stable, small patch.