What labs are used to monitor hyperthyroidism?

Laboratory Tests for Monitoring Hyperthyroidism

The primary laboratory tests for monitoring hyperthyroidism are thyroid stimulating hormone (TSH), free thyroxine (free T4), and free triiodothyronine (free T3). 1, 2

Initial Diagnostic Testing

When hyperthyroidism is suspected, the following tests should be ordered:

1. Thyroid Stimulating Hormone (TSH)

   • Most sensitive initial screening test for thyroid dysfunction 3
   • Low or suppressed in hyperthyroidism
   • Normal range: typically 0.4-4.5 mIU/L (lab-specific)

2. Free Thyroxine (Free T4)

   • Elevated in overt hyperthyroidism
   • Helps distinguish between subclinical and overt hyperthyroidism 3
   • Normal in subclinical hyperthyroidism

3. Free Triiodothyronine (Free T3)

   • Important to measure, especially in T3-toxicosis where only T3 is elevated 4
   • May be elevated even when Free T4 is normal

Diagnostic Patterns in Hyperthyroidism

• Overt hyperthyroidism: Suppressed TSH with elevated Free T4 and/or Free T3 1
• Subclinical hyperthyroidism: Suppressed TSH with normal Free T4 and Free T3 5
• T3-toxicosis: Suppressed TSH, normal Free T4, elevated Free T3 4

Additional Testing for Etiology

Once hyperthyroidism is confirmed, additional tests help determine the cause:

1. Thyroid Antibodies

   • TSH receptor antibodies (TRAb) - for Graves' disease 5
   • Thyroid peroxidase antibodies (TPO-Ab) - often present in autoimmune thyroid disease 3
   • Thyroglobulin antibodies (TG-Ab) - may be present in autoimmune thyroid disease 3

2. Imaging Studies (when etiology is unclear)

   • Thyroid ultrasound - to evaluate for nodules or diffuse enlargement
   • Radioactive iodine uptake scan - distinguishes between destructive thyroiditis and hyperthyroidism due to Graves' disease or nodular goiter 5

Monitoring Protocol

Frequency of Testing

• Initial diagnosis: Complete thyroid panel (TSH, Free T4, Free T3)
• During treatment initiation: Monitor every 2-4 weeks until stable 5
• Maintenance phase: Every 3-6 months once stable 5
• After treatment discontinuation: Monitor every 1-3 months for the first year to detect recurrence 1

Special Monitoring Considerations

1. Patients on Antithyroid Drugs (e.g., Methimazole)

   • Monitor thyroid function tests periodically during therapy 6
   • The goal is to maintain Free T4 or Free T4 Index in the high-normal range using the lowest possible thioamide dosage 5
   • A rising serum TSH indicates that a lower maintenance dose should be employed 6

2. Pregnancy

   • More frequent monitoring (every 2-4 weeks) is recommended 5
   • Target Free T4 in the high-normal range for pregnant women 5

3. Subclinical Hyperthyroidism

   • If TSH is between 0.1-0.45 mIU/L: Repeat testing within 3 months 5
   • If TSH is <0.1 mIU/L: Repeat measurement, along with Free T4 and Free T3, within 4 weeks 5

Common Pitfalls in Laboratory Testing

1. Non-thyroidal illness syndrome: Acute illness can affect thyroid tests without true thyroid dysfunction

2. Medication effects: Amiodarone, glucocorticoids, dopamine can alter thyroid function tests

3. Laboratory interference: Biotin supplements can interfere with some thyroid assays

4. Relying solely on TSH: In early hyperthyroidism treatment, TSH may remain suppressed for months after the patient becomes euthyroid 7

5. Overlooking T3-toxicosis: Some patients have normal Free T4 but elevated Free T3 4, 8

By following this structured approach to laboratory monitoring, clinicians can effectively diagnose, treat, and monitor patients with hyperthyroidism while avoiding common diagnostic pitfalls.