Does heparin (anticoagulant) cause low Thyroid-Stimulating Hormone (TSH) levels?

Heparin Can Cause Artificially Low TSH Levels in Laboratory Testing

Heparin administration can cause artificially low TSH measurements due to in vitro effects on thyroid hormone binding, which is an important laboratory artifact to recognize when interpreting thyroid function tests. 1

Mechanism of Heparin's Effect on Thyroid Function Tests

• Heparin (both unfractionated and low-molecular-weight) releases lipoprotein lipase, which generates free fatty acids in vitro that displace thyroid hormones from their binding proteins 1
• This leads to artificially elevated free thyroid hormone (FT3, FT4) measurements and a corresponding decrease in TSH levels 2
• The drop in TSH is likely a physiological response to the artificially elevated free thyroid hormone levels rather than a direct effect of heparin on TSH production 2

Laboratory Findings Associated with Heparin Administration

• Elevated free T3 and free T4 levels in blood samples collected with heparin 3
• Decreased TSH levels that appear to be a response to the elevated thyroid hormone measurements 2
• These changes represent laboratory artifacts rather than true alterations in thyroid function 1

Clinical Significance

• These laboratory abnormalities can lead to diagnostic confusion and unnecessary investigations if not recognized 1
• The effect is particularly important to recognize in hospitalized patients who are receiving heparin therapy 4
• The phenomenon occurs with both unfractionated heparin and low-molecular-weight heparins 1

Recommendations for Clinical Practice

• When interpreting thyroid function tests in patients receiving heparin therapy, consider the possibility of laboratory interference 4
• If possible, collect blood samples for thyroid function testing before initiating heparin therapy 1
• If abnormal thyroid function tests are found in a patient on heparin therapy without clinical signs of thyroid dysfunction, repeat testing after discontinuation of heparin when feasible 1
• Use non-heparinized collection tubes when possible for thyroid function tests in patients receiving heparin therapy 3

Important Distinctions

• This effect is different from true thyroid dysfunction and represents an in vitro artifact 1
• While heparin can affect laboratory measurements, there is no evidence that it causes actual thyroid dysfunction in vivo 4
• Interestingly, there is some evidence suggesting that hyperthyroidism may increase unfractionated heparin requirements, which is a separate phenomenon from the laboratory interference described above 5