TSH as an index of L-Thyroxine replacement and suppression therapy

Summary

When hypothalamic-pituitary function is normal, serum TSH levels measured by ultrasensitive assay yield bioassays of endogenous thyroid action and thus provide an ideal index of thyroid secretion and its relationship to fluctuating endogenous thyroid levels. It is theoretically possible that patients receiving exogenous L-thyroxine for primary hypothyroidism should have suppressed TSH levels if physiological needs are constantly met. To examine this possibility free thyroxine, FT4 and TSH were measured in 90 clinically euthyroid patients receiving treatment with L-thyroxine for primary hypothyroidism. TSH levels were normal in 44, suppressed in 16 and elevated in 30 patients. FT4 levels were normal in 68, elevated in 13 and suppressed in 9 patients. Normal TSH levels were associated with normal FT4 levels in 79.5% of patients, elevated FT4 levels in 13.6% and low FT4 in 6.8%. Suppressed TSH levels were associaed with elevated FT4 levels in 37.5% of patients and normal FT4 levels in 62.5%. When FT4 levels were normal, however, TSH levels were normal in only 51.5% and abnormal in 48.5%. We also examined the possibility that FT4 levels may remain within normal range when TSH is suppressed during L-thyroxine treatment for goitre or cancer. FT4 and TSH were measured in 45 patients on L-thyroxine as TSH suppression treatment. TSH was suppressed in 23 patients (51.1%), normal in 20 (44.4%) and elevated in 2 (4.5%). When TSH was suppressed, FT4 was elevated in 30.4% but normal in 69.6% of patients. In summary, these observations do not support the hypothesis that a suppressed TSH level indicates optimal replacement in patients with primary hypothyroidism, rather that TSH levels should be normal. Where TSH suppression is required this can be achieved in approximately 70% of patients without elevating the Free T4 level.