['We examined the interrelationships of pituitary thyrotropin (TSH) with circulating thyroid hormones to determine whether they were expressed either invariably or conditionally and distinctively related to influences such as levothyroxine (L-T4) treatment.', 'This prospective study employing 1912 consecutive patients analyses the interacting equilibria of TSH and free triiodothyronine (FT3) and free thyroxine (FT4) in the circulation.', 'The complex interrelations between FT3, FT4 and TSH were modulated by age, body mass, thyroid volume, antibody status and L-T4 treatment. By group comparison and confirmation by more individual TSH-related regression, FT3 levels were significantly lower in L-T4-treated vs untreated nonhypothyroid autoimmune thyroiditis (median 4·6 vs 4·9\xa0pm, P\xa0<\xa00·001), despite lower TSH (1·49 vs 2·93\xa0mU/l, P\xa0<\xa00·001) and higher FT4 levels (16·8 vs 13·8\xa0pm, P\xa0<\xa00·001) in the treated group. Compared with disease-free controls, the FT3-TSH relationship was significantly displaced in treated patients with carcinoma, with median TSH of 0·21 vs 1·63 (P\xa0<\xa00·001) at a comparable FT3 of 5·0\xa0pm in the groups. Disparities were reflected by calculated deiodinase activity and remained significant even after accounting for confounding influences in a multivariable model.'] Are homeostatic equilibria between free thyroid hormones and pituitary thyrotropin modulated by various influences including age , body mass index and treatment?