6) http:/// pubmed/1366242
Ir J Med Sci. 1992 Dec;161(12):684-6. TSH as an index of L-thyroxine replacement and suppression therapy. Igoe D1, Duffy MJ, McKenna TJ.
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 % of patients, elevated FT4 levels in % and low FT4 in %. Suppressed TSH levels were associated with elevated FT4 levels in % of patients and normal FT4 levels in %. When FT4 levels were normal, however, TSH levels were normal in only % and abnormal in %. 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 (%), normal in 20 (%) and elevated in 2 (%). When TSH was suppressed, FT4 was elevated in % but normal in % of patients.(ABSTRACT TRUNCATED AT 250 WORDS)