Serum thyrotropin (TSH) levels in patients with suppressed pituitary function

The diagnosis of borderline hyperthyroidism is difficult. A sensitive radioimmunoassay capable of detecting subnormal levels of serum TSH may be of value in confirming this diagnosis because of the suppressed pituitary function in this disease state. This sensitive assay may also be useful in monitoring the suppression of pituitary function in thyroid cancer patients receiving thyroid hormone therapy. A sensitive radioimmunoassay capable of detecting serum TSH levels as low as 0.25 ..mu..U/m1 with coefficients of variation less than 17.2% was used to measure serum TSH levels in 80 healthy subjects, 44 hyperthyroid patients, and 25 athyrotic thyroid cancer patients on daily suppressive doses of thyroxine. All healthy subjects had detectable TSH levels with a mean value of 1.17 and two standard deviation ranges of 0.41 - 2.70 ..mu..U/m1 (lognormal distribution). Although the mean +-1 SEM value of 0.63 +- 0.003 ..mu..Um1 for hyperthyroid patients and 0.76 +- 0.08 ..mu..U/ml for thyroid cancer patients were significantly lower than that of healthy subjects (t-test, p<0.05), subnormal levels of serum TSH were found in only 28.6% (12/42) and 24% (6/25) of hyperthyroid and thyroid cancer patients, respectively. TSH stimulation tests performed in 6 of the cancer patients all gave suppressed responses. Because ofmore » considerable overlap, serum TSH levels alone cannot distinguish hyperthyroidsm from euthyroidism. However, a sensitive TSH radioimmunoassay such as the one described here may be of value in evaluating the extent of pituitary suppression in thyroid cancer therapy.« less