Differential Diagnosis for a 17-year-old with Psoriasis and Laboratory Results

Single Most Likely Diagnosis

• Psoriatic Arthritis with Thyroid Dysfunction: The presence of psoriasis, a positive MCV antibody (often associated with rheumatoid arthritis but can be seen in other autoimmune diseases like psoriatic arthritis), and abnormal thyroid function tests (elevated thyroid peroxidase antibodies, low TSH, and low free T4) suggest an autoimmune component. Psoriatic arthritis is a common comorbidity in patients with psoriasis and can be associated with other autoimmune conditions.

Other Likely Diagnoses

• Rheumatoid Arthritis (RA): Although less likely given the context of psoriasis, the positive MCV antibody is a strong indicator for RA. However, the clinical presentation and other autoimmune features (like thyroid dysfunction) might lean more towards psoriatic arthritis.
• Hashimoto's Thyroiditis: The elevated thyroid peroxidase antibodies and abnormal thyroid function tests (low free T4 and low TSH, which might indicate a complex thyroid dysfunction picture) are consistent with Hashimoto's thyroiditis, an autoimmune thyroid disease.
• Autoimmune Polyendocrine Syndrome (APS): Given the combination of autoimmune thyroid disease (suggested by thyroid peroxidase antibodies and thyroid function tests) and another autoimmune condition (psoriasis, potentially with psoriatic arthritis), APS could be considered, although it is less common.

Do Not Miss Diagnoses

• Graves' Disease: Although the TSH is low, which could suggest hyperthyroidism, the free T4 is also low, making the interpretation complex. Graves' disease is an autoimmune condition that can present with low TSH and can be life-threatening if not treated properly.
• Thyroid Storm: Any condition that leads to severe thyroid dysfunction can potentially lead to a thyroid storm, a life-threatening condition requiring immediate medical attention.

Rare Diagnoses

• Mixed Connective Tissue Disease (MCTD): This is an autoimmune disease that can present with features of lupus, scleroderma, and rheumatoid arthritis. While it's rare, the combination of autoimmune thyroid disease, positive MCV antibodies, and psoriasis could potentially fit into a broader autoimmune syndrome like MCTD.
• Sjögren's Syndrome: Another autoimmune disease that could potentially explain some of the findings, especially if there are other symptoms like dry eyes or mouth, although it's less directly related to the specific lab findings presented.