Seborrheic Dermatitis: This condition is characterized by a dry, itchy scalp and can be exacerbated by environmental changes, such as moving to a new location with different climate conditions. The relief found with antihistamines and coal tar shampoo supports this diagnosis, as these treatments are commonly used for seborrheic dermatitis.

Atopic Dermatitis: Given the response to antihistamines, atopic dermatitis (eczema) is a possible diagnosis. It's an allergic condition that can cause dryness and itchiness on the scalp.
Psoriasis: Although less likely without other characteristic symptoms like thick, scaly plaques, psoriasis can cause an itchy scalp. Coal tar shampoo, which was beneficial, is sometimes used in treating psoriasis.
Contact Dermatitis: An allergic reaction to a new hair product or environmental factor in Calgary could cause an itchy scalp. The improvement with antihistamines and a specific shampoo could indicate a reaction to something that was avoided or replaced.

Tinea Capitis (Ringworm of the Scalp): Although less common, fungal infections like tinea capitis can cause itchy scalps. It's crucial to diagnose and treat this condition to prevent spread and potential complications.
Scalp Lymphoma or Other Malignancies: While rare, any persistent or worsening scalp condition should prompt consideration of malignancies. Early detection is critical for effective treatment.

Pityriasis Amiantacea: A rare condition characterized by a thick, scaly, and itchy scalp. It's often associated with psoriasis or seborrheic dermatitis.
Keratosis Pilaris: Although more commonly found on arms and legs, keratosis pilaris can occasionally affect the scalp, causing dryness and itchiness.