title: Homeostatic equilibria between free thyroid hormones and pituitary thyrotropin are modulated by various influences including age, body mass index and treatment.
abstract: 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.
  
  
  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. 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 pm, P < 0·001), despite lower TSH (1·49 vs 2·93 mU/l, P < 0·001) and higher FT4 levels (16·8 vs 13·8 pm, P < 0·001) 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 < 0·001) at a comparable FT3 of 5·0 pm in the groups. Disparities were reflected by calculated deiodinase activity and remained significant even after accounting for confounding influences in a multivariable model.
  
  
  CONCLUSIONS
  TSH, FT4 and FT3 each have their individual, but also interlocking roles to play in defining the overall patterns of thyroidal expression, regulation and metabolic activity. Equilibria typical of the healthy state are not invariant, but profoundly altered, for example, by L-T4 treatment. Consequently, this suggests the revisitation of strategies for treatment optimization.
authors: Department of Nuclear Medicine, Klinikum Luedenscheid, Luedenscheid, Germany.
authors: Rudolf
authors: R
authors: Hoermann
authors: John E M
authors: JE
authors: Midgley
authors: Adrienne
authors: A
authors: Giacobino
authors: Walter A
authors: WA
authors: Eckl
authors: Hans Günther
authors: HG
authors: Wahl
authors: Johannes W
authors: JW
authors: Dietrich
authors: Rolf
authors: R
authors: Larisch
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D015992:Body Mass Index; D005260:Female; D006706:Homeostasis; D020249:Hormone Replacement Therapy; D006801:Humans; D006980:Hyperthyroidism / Q000378:metabolism*; D007037:Hypothyroidism / Q000188:drug therapy / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D013959:Thyroid Diseases / Q000378:metabolism; D013961:Thyroid Gland / Q000378:metabolism*; D013972:Thyrotropin / Q000378:metabolism*; D013974:Thyroxine / Q000378:metabolism* / Q000627:therapeutic use; D014284:Triiodothyronine / Q000378:metabolism*
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine