Monitor Free T3 in Graves' Disease
For monitoring Graves' disease during treatment, free T3 (FT3) should be measured alongside free T4 (FT4) and TSH, as FT3 correlates more closely with metabolic status and clinical thyroid function than FT4 alone during antithyroid drug therapy. 1, 2
Rationale for Free T3 Monitoring
Free T3 shows the strongest correlation with basal metabolic rate (BMR) during antithyroid drug treatment in Graves' disease patients (r = 0.6088, P < 0.001), superior to T4, RT3U, or free thyroxine index. 2
The FT3/FT4 ratio remains relatively stable in Graves' disease patients as thyroid hormone levels change during treatment, whereas this ratio decreases in destructive thyroiditis, making it useful for differential diagnosis and monitoring. 3, 4
Graves' disease characteristically produces disproportionately elevated T3 relative to T4 due to preferential T3 secretion from the hyperstimulated thyroid gland. 3, 4
Comprehensive Monitoring Protocol
Measure FT4 and FT3 initially during active hyperthyroidism, then continue monitoring both FT4, FT3, and TSH throughout antithyroid drug therapy. 1
TSH remains suppressed for extended periods even after thyroid hormone levels normalize, making it an unreliable sole marker during early treatment phases. 5
TSH may better reflect circulating thyroid-stimulating immunoglobulin (TSI) levels than actual thyroid function, as TSH can remain suppressed despite normal or even low FT4 and FT3 concentrations. 5
Clinical Application During Treatment
When FT4 levels are elevated (>3.9 ng/dL), the FT3/FT4 ratio becomes increasingly useful for confirming Graves' disease versus thyroiditis, with sensitivity reaching 77.8-97.0% at higher FT4 levels. 3
An FT3/FT4 ratio >4.4 (×10⁻² pg/ng) offers 92.8% specificity for Graves' disease, though sensitivity is only 47.2%, making it most useful when elevated. 4
During antithyroid drug treatment, the normal range of FT3 (94.2-184.0 ng/dL) corresponds better to normal BMR than the corresponding normal ranges for FT4 or other thyroid indices. 2
Monitoring Algorithm
Initial assessment: Measure TSH, FT4, FT3, and thyroid-stimulating antibodies (TSI or TRAb) before starting treatment. 1
During active treatment: Monitor FT4 and FT3 every 2-4 weeks initially, as these reflect current thyroid status more accurately than TSH. 6, 1
As treatment progresses: Add TSH to monitoring once FT4 and FT3 normalize, checking all three parameters together. 1
Before discontinuing therapy: Recheck TSI/TRAb levels, as persistently elevated antibodies predict relapse risk. 1
Critical Pitfalls to Avoid
Do not rely on TSH alone during active treatment or early recovery, as it remains suppressed despite improving thyroid hormone levels and does not correlate well with clinical thyroid function. 5
Do not assume normal FT4 means adequate control—FT3 may remain elevated (T3-toxicosis) and requires specific monitoring. 1, 2
Avoid mistaking recovery-phase thyroiditis for inadequately treated Graves' disease by checking the FT3/FT4 ratio, which decreases in thyroiditis but remains stable in Graves' disease. 3