Do quetiapine and gabapentin affect Thyroid-Stimulating Hormone (TSH) levels?

Quetiapine and Gabapentin Effects on TSH Levels

Quetiapine can significantly alter TSH levels and cause thyroid dysfunction, while gabapentin has no documented effect on thyroid function. 1

Quetiapine's Effects on Thyroid Function

Evidence from Drug Label

• According to the FDA label for quetiapine, the medication can cause significant changes in thyroid function tests 1:
  • 3.2% of patients on quetiapine experience TSH increases (>5 mIU/L) compared to 2.7% on placebo
  • 0.7% of patients show decreases in free T4 levels
  • Some patients with elevated TSH required thyroid replacement therapy
  • In mania adjunct studies, 12% of quetiapine-treated patients had elevated TSH levels compared to 7% of placebo-treated patients

Mechanism and Clinical Implications

• The exact mechanism by which quetiapine affects thyroid function is unclear 1
• Quetiapine primarily causes decreases in total T4 levels 2
• These changes may be related to competitive metabolism of thyroid hormones and quetiapine by UDP-glucuronosyltransferase 2
• Both immunological and non-immunological mechanisms may be involved 3

Clinical Course of Quetiapine-Induced Thyroid Changes

• Thyroid abnormalities typically occur within weeks to months after starting quetiapine 4, 5
• In some cases, thyroid function may normalize spontaneously without discontinuing quetiapine 5
• In nearly all cases, cessation of quetiapine treatment leads to reversal of effects on T4 levels 1

Gabapentin's Effects on Thyroid Function

• There is no evidence in the provided literature indicating that gabapentin affects TSH or other thyroid parameters
• None of the guidelines or research papers mention gabapentin as causing thyroid dysfunction
• Gabapentin is not listed among psychotropic medications known to affect thyroid function 6

Monitoring Recommendations

For Patients on Quetiapine

• Both TSH and free T4 should be measured at baseline and at follow-up visits 1
• TSH alone may not accurately reflect thyroid status in patients taking quetiapine 1
• Patients with pre-existing thyroid conditions may be at higher risk for developing hypothyroidism when taking quetiapine 4

For Patients on Gabapentin

• No specific thyroid function monitoring is recommended for gabapentin based on available evidence

Important Clinical Considerations

• TSH levels can vary by up to 50% day-to-day, so serial measurements provide more reliable information than a single test 7
• TSH secretion is affected by many factors beyond thyroid disorders, including acute illness and various medications 8
• The reference range for TSH (generally 0.4-4.5 mIU/L) varies among different populations, with older adults often having higher baseline TSH levels 8, 7

Common Pitfalls to Avoid

• Relying on a single abnormal TSH value to diagnose thyroid dysfunction in patients on quetiapine
• Failing to measure both TSH and free T4 in patients taking quetiapine
• Overlooking that quetiapine-induced thyroid changes may resolve spontaneously without discontinuation in some cases
• Attributing thyroid abnormalities to medications when other causes may be present

In summary, while quetiapine has well-documented effects on thyroid function that warrant monitoring, there is no evidence suggesting gabapentin affects TSH or thyroid function.