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Is your patient having persistently high TSH , despite increasing the dose of your thyroid medications ? The patient on the other hand is asymptomatic and the T4/Free T4 values are always normal. Contd..

High TSH despite adequate LT4 replacement is a common clinical issue that we deal with in Endocrinology and it is an area of several #ClinicalPearl threads

However today we focus on one such issue on possible reason for the problem. The reason is what is called a Heterophile antibody

In patients with other autoimmune disease having hypothyroidism with persistently high TSH it is a great idea to look for RA factor

RA factor is a form of heterophile antibody that interferes with the immunoassay resulting in false high results

There are many other reasons for heterophile antibodies, another common etiology is the presence of HAMA- Human antimouse antibodies.

This interesting case illustrates this issue. The patient came with recalcitrant TSH and elevation in other Endocrine lab results. The patient had high titers of RA factor https://doi.org/10.3109/00365513.2016.1143113

There are several ways to confirm the interference with Heterophile antibodies. Some commercial assay platforms have blockers for the same

However, perhaps the simplest method is to perform a serial dilution of the sample. In presence of this interfering antibodies, linearity is NOT maintained in serial dilution

The issue with heterophile antibodies is restricted to larger molecules- TSH, HCG, PSA, LH, FSH etc and results are always false positive or false high

In summary- Suspect Heterophile antibodies in patients having likely false high or false-positive results especially those with other autoimmune diseases and/or having RA factor positive in high titers