Patterns of primary thyroid failure

Serum thyroxine(T4), triiodothyronine (T3) and thyroid-stimulating hormone (TSH) levels were measured in 170 patients with suspected hypothyroidism. Only patients with elevated serum TSH levels indicative of some degree of primary thyroid failure were included in the study. They appeared to represent four distinct classes of thyroid failure: class 1 (19.5%) had normal T4 and normal T3; class 2 (8%) had low T4 and elevated T3; class 3 (37%) had low T4 and normal T3; and class 4 (35.5%) had low T4 and low T3. TSH levels were increased in all classes. Of 10 patients who were followed without treatment and evaluated after three and six months, six showed increasing clinical and chemical evidence of hypothyroidism and were reclassified, two did not progress and two patients improved. Our study showed that serum T3 levels may be normal or even elevated in hypothyroidism and, therefore, are not a useful indicator. The data also suggested that serum T4 is the main determinant of the metabolic state of the patient, since class 3 patients, the largest single group, had normal serum T3 levels despite striking clinical evidence of hypothyroidism. Thus, a normal serum T3 in the absence of a normal serum T4 is not generally sufficient for maintenance of a euthyroid state. Finally, our study showed that in primary thyroid failure there is better reciprocal correlation between the levels of serum TSH and T4 than between those of serum TSH and T3.