The clinical value of serum triiodothyronine, thyroxine and thyrotropin estimations during medical antithyroid treatment.

The relation between clinical status and serial determinations of serum T3, serum T4 and serum TSH has been evaluated in the early phase of medical antithyroid treatment in 12 unselected hyperthyroid patients, and in 19 patients who later during treatment accidentally developed low serum T4 values. Determination of both serum T3 and serum T4 was found necessary to avoid undertreatment. Two patients with signs of hypothyroidism in the early phase developed low serum T4, while serum T3 and serum TSH remained normal. In all of the 19 patients selected with low serum T4, serum T3 was normal. Serum TSH was elevated in 5 patients without hypothroid symptoms, while 2 developed hypothyroid symptoms in spite of normal serum TSH values. Our results suggest that serum T4 is a more sensitive parameter than both serum TSH and serum T3 in avoiding overtreatment during medical antithyroid treatment.