Differential diagnosis for high TSH and low free T4 with ICPI therapy

What is the differential diagnosis for an elevated serum thyroid-stimulating hormone (TSH) and a normal or low serum free T in the setting of ICPI therapy?

Primary hypothyroidism should be suspected in the setting of an elevated serum TSH level with a normal or low serum free T value; however, this thyroid test pattern can also be seen in the hypothyroid phase of destructive thyroiditis. An enlarged thyroid gland and positive thyroid antibodies (thyroid peroxidase and thyroglobulin antibodies) suggests autoimmune hypothyroidism, whereas a normal gland size and negative antibodies would favor thyroiditis. When findings are equivocal, follow-up laboratory examinations in 3 to 4 weeks may help determine if a recovering thyroiditis is present.


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