In this article, we report on two cases of discordant thyroid function test results, something which may occur at all physicians’ practices. The first patient is a 55-year-old male with atrial fibrillation and pulmonary embolism who was treated with heparin. The diagnostic work-up for atrial fibrillation revealed that the serum level of thyroid stimulating hormone (TSH) was normal but that the free T4 was elevated. The elevated free T4 level appeared to have been caused by heparin, which causes the release of T4 that is bound to plasma proteins. After the discontinuation of heparin therapy, the free T4 level returned to normal. The second patient, a 36-year-old female, showed elevated TSH levels but normal free T4 levels and was diagnosed with Sjögren’s syndrome. In this case, the rheumatoid factor in the patient’s plasma had interfered with the immunoassay for TSH, which resulted in falsely elevated results. Removal of the interfering antibodies resulted in normal TSH test results.
Conflict of interest: none declared. Financial support: none declared.