Homeostatic equilibria between free thyroid hormones and pituitary thyrotropin are modulated by various influences including age, body mass index and treatment.

Objective: 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.

Design And Methods: This prospective study employing 1912 consecutive patients analyses the interacting equilibria of TSH and free triiodothyronine (FT3) and free thyroxine (FT4) in the circulation.

Results: The complex interrelations between FT3, FT4 and TSH were modulated by age, body mass, thyroid volume, antibody status and L-T4 treatment.