An assessment of plasma TSH radioimmunoassay and of the TSH stimulation test in the diagnosis of 100 consecutive patients with suspected hypothyroidism.

SUMMARY A plasma TSH estimation and a TSH stimulation test were performed on each of 100 consecutive patients seen at a thyroid clinic in whom it was necessary to exclude a diagnosis of hypothyroidism. None of the patients had received replacement therapy. The normal response to 10 i.u. bovine TSH administered intramuscularly on three consecutive days was shown to be an absolute increment in the 4 hr uptake of 132I by the thyroid gland of at least 13%. The mean plasma TSH in twenty-five untreated euthyroid patients, who had neither goitres nor thyroid antibodies, whose TSH stimulation tests were normal and in whom there was no evidence of hypopituitarism, was 3.8 μU/ml+3.6 (2 S.D.). All forty-nine patients with primary hypothyroidism had plasma TSH levels in excess of 25 μU/ml. In eighty-nine of the 100 patients, the radioimmunoassay of plasma TSH and the TSH stimulation test gave similar information about the reserve of thyroid function and in five euthyroid patients the plasma TSH was > 7.4 μU/ml while the TSH stimulation test was normal. The remaining six subjects (including one patient with hypopituitarism and three who had been treated with radioiodine for thyrotoxicosis) were euthyroid with an abnormal TSH stimulation test and a plasma TSH of <7.4 μU ml. For clinical purposes a plasma TSH of 25 μU/ml indicates an impaired or absent reserve of thyroid function with or without hypothyroidism. It is concluded that a single plasma TSH estimation should replace the time-consuming TSH stimulation test in the assessment of thyroid reserve.