Serum thyrotropin in Graves' disease: a more reliable index of circulating thyroid-stimulating immunoglobulin level than thyroid function?

OBJECTIVE

To assess the relationship between serum thyrotropin (thyroid-stimulating hormone or TSH) on one hand and thyroid-stimulating immunoglobulin (TSI), free thyroxine (T4), and triiodothyronine (T3) levels on the other in Graves' disease, inasmuch as TSH may be suppressed in the presence of TSI because TSI may bind to the TSH receptor on the thyroid gland membrane and thus eliminate the need for circulating TSH for stimulating the thyroid gland.

METHODS

We determined serum TSI levels in 37 women and 13 men with Graves' disease, stratified into 4 groups on the basis of serum TSH levels irrespective of serum free T4 and T3 levels. Our reference ranges were 0.72 to 1.74 ng/dL for free T4, 80 to 200 ng/dL for T3, and 0.4 to 4.0 micro/mL for TSH.

RESULTS

Mean serum TSI concentrations were highest (215% +/- 28%) in patients with undetectable TSH levels (<0.03 micro/mL) and lowest (103% +/- 9%) in those with supernormal TSH concentrations (>4.0 micro/mL). TSI levels were intermediate in the other study groups: 157% +/- 16% in patients with subnormal though detectable TSH levels (0.03 to 0.39 micro/mL) and 125% +/- 12% in those with normal TSH levels (0.4 to 4.0 micro/mL). Moreover, a progressive decline in TSI levels with increasing serum TSH concentrations was noted, along with a significant negative correlation (r = -0.45; P<0.01) between serum TSI and TSH concentrations. Finally, relationships between free T4 and T3 levels on one hand and TSI or TSH levels on the other were not significant, with a considerable variability in free T4 and T3 levels being noted in individual study groups.

CONCLUSION

Serum TSH is frequently suppressed after treatment with antithyroid drugs or radioiodine (131I), irrespective of clinical thyroid function as expressed by increased, normal, or decreased free T4 and T3 concentrations. In an individual patient with Graves' disease, the serum TSH level may be more reflective of the circulating TSI concentration than is thyroid gland function as expressed by free T4 and T3 concentrations and therefore may be as reliable a predictor of remission as TSI.