The relationship between levels of thyroid stimulating hormone and of thyroxine and triiodothyronine in blood of hypothyroid patients.

Serum TSH values and the free concentration of T4 and T3 in serum expressed as the free T4- and T3-indices showed a curvilinear relationship in 24 patients with manifest hypothyroidism mainly due to extremely elevated serum TSH levels associated with very low thyroid hormone levels. The same degree of negative correlation was found between the log serum TSH concentration and serum free T4-index (r = -0.51, P less than 0.05) and between log serum TSH concentration and serum free T3-index (r = -0.47, P less than 0.05). The correlation coefficients dropped to non-significant levels when the interdependence between the free T3- and T4-indices was taken into account and eliminated by partial correlation analysis. The degree of negative correlation between serum TSH and the thyroid hormones was also of the same order of magnitude in 5 hypothyroid patients studied prosepctively during treatment with L-thyroxine (TSH vs. T4: mean r = -0.71; TSH vs T3: mean r = -0.79).