Additional Labs to Order When TSH is Low
When TSH is low, you should order Free T4 (FT4) and Free T3 (FT3) to properly assess thyroid function and determine the cause of the low TSH. 1
Diagnostic Algorithm for Low TSH
Initial Finding: Low TSH (<0.45 mIU/L)
Essential Follow-up Labs:
- Free T4 (FT4)
- Free T3 (FT3)
Interpretation of Results:
- Low TSH + High FT4/FT3: Overt hyperthyroidism
- Low TSH + Normal FT4/FT3: Subclinical hyperthyroidism
- Low TSH + Low/Normal FT4/FT3: Consider central hypothyroidism or non-thyroidal illness
Additional Testing Based on Clinical Suspicion
After obtaining FT4 and FT3, further testing may be indicated:
For suspected autoimmune thyroid disease:
- Thyroid peroxidase antibodies (TPOAb)
- Thyroid stimulating immunoglobulins (TSI) or TSH receptor antibodies (TRAb) if Graves' disease is suspected
For suspected nodular disease:
- Thyroid ultrasound (especially if physical exam reveals nodules)
- Thyroid uptake scan (particularly useful to identify hot nodules) 2
Monitoring Recommendations
For patients with low TSH, the Endocrine Society recommends the following monitoring frequency 1:
- TSH 0.1-0.45 mIU/L: Recheck every 3 months
- TSH <0.1 mIU/L: Recheck every 4-6 weeks
Important Considerations
Avoid ordering unnecessary panels: Research shows that comprehensive thyroid panels (TSH+T4+T3+FT4+FT3) are often ordered unnecessarily. A more targeted approach starting with TSH followed by free hormone levels is more cost-effective 3.
Consider interference factors: When there's a discrepancy between TSH and free hormone levels, consider potential assay interferences such as:
- Thyroid hormone autoantibodies
- Heterophilic antimouse antibodies
- Abnormal albumin variants 4
Clinical context matters: Low but detectable TSH in ambulatory patients often indicates underlying thyroid disease, commonly multinodular goiter or autonomous nodules, even when free hormone levels are normal 2.
Treatment decisions: For subclinical hyperthyroidism, treatment is recommended for patients >65 years old with TSH <0.1 mIU/L or those with risk factors for cardiovascular disease or osteoporosis 1.
Common Pitfalls to Avoid
Relying solely on TSH: While TSH is a sensitive marker, free hormone levels provide critical information about peripheral thyroid status 5.
Ignoring clinical symptoms: Laboratory results should always be interpreted in the context of the patient's clinical presentation.
Missing central hypothyroidism: Low TSH with low/normal free hormone levels may indicate pituitary or hypothalamic dysfunction rather than hyperthyroidism.
Overlooking non-thyroidal illness: Acute illness can suppress TSH without true hyperthyroidism.