Dry Skin (Xerosis): This is a common condition, especially in older adults, characterized by dry, itchy skin. It's poorly responsive to antihistamines like hydroxyzine because its pathophysiology is not primarily driven by histamine release.

Atopic Dermatitis (Eczema): Although hydroxyzine can be used to treat the pruritus associated with atopic dermatitis, severe cases may not respond adequately, necessitating the use of topical corticosteroids or other treatments.
Psoriasis: Pruritus can be a symptom of psoriasis, and while antihistamines may provide some relief, they are not the primary treatment for this condition.
Urticaria: Chronic urticaria can cause pruritus, but if it's not responding well to hydroxyzine, other causes or additional treatments should be considered.
Hypothyroidism: This condition can cause dry, itchy skin that may not respond well to antihistamines.

Hodgkin Lymphoma: Pruritus can be an early symptom of Hodgkin lymphoma, and it's crucial to investigate this possibility, especially if other symptoms like weight loss, fever, or lymphadenopathy are present.
Cholestasis: Intrahepatic or extrahepatic cholestasis can cause severe pruritus that is not responsive to antihistamines. This condition requires prompt diagnosis and treatment to prevent complications.
Multiple Myeloma: Pruritus can be associated with multiple myeloma, and given the potential severity of this condition, it should not be missed.
Thyroid Disorders (Hyperthyroidism): While hypothyroidism is more commonly associated with dry skin, hyperthyroidism can also cause pruritus due to increased sensitivity to histamine and other mechanisms.

Polycythemia Vera: A myeloproliferative disorder that can cause pruritus, especially after hot showers, due to the release of histamine from basophils.
Neurological Disorders (e.g., Multiple Sclerosis, Neuropathies): Certain neurological conditions can cause pruritus without a clear dermatological cause, often referred to as neuropathic pruritus.
Paraneoplastic Syndromes: Rarely, pruritus can be a paraneoplastic symptom of various cancers, making it essential to investigate for underlying malignancies in some cases.