Three-day and single-dose thyroid-stimulating hormone (TSH) tests were performed in 211 subjects with suspected or known thyroid hypofunction. Both procedures differentiated between euthyroidism and hypothyroidism. The mean changes in 24-hr 131I uptake after 3-day dosage with TSH and in 3-hr 131I uptake after one dose of TSH were significantly greater in euthyroid than in hypothyroid subjects. Comparable separation was afforded by concomitant measurement of plasma protein-bound iodine (PBI) before and after TSH administration with either the three-dose or single-dose procedure. Use of 10 units rather than 5 units TSH in the single-dose procedure did not increase its diagnostic sensitivity.
The single-dose TSH test was valuable in characterizing euthyroidism in subjects receiving thyroid hormone because of an unconfirmed and erroneous prior diagnosis of hypothyroidism.
Laboratory diagnosis of "low thyroid reserve" does not preclude subsequent long-standing euthyroidism as demonstrated in 10 to 12 patients so diagnosed and followed for 2 to 6 years.
TSH testing of euthyroid subjects with multi-nodular goiter showed relatively impaired 131I uptake response, although normal post-TSH PBI increase was observed.
Although the 3-day TSH test distinguished hypopituitary patients from those with primary hypothyroidism, neither 5-unit nor 10-unit single-dose TSH tests permitted differentiation between these two patient groups.