title: Serum free triiodothyronine (T3) to free thyroxine (T4) ratio in treated central hypothyroidism compared with primary hypothyroidism and euthyroidism.
abstract: The standard treatment of hypothyroidism (central and primary) consists of thyroxine (T4) administration alone. However, the normal thyroid gland produces a small proportion of triiodothyronine (T3) directly into the circulation.
  
  
  OBJECTIVE
  We aimed to study the free T3 to free T4 ratio in treated central hypothyroidism compared with euthyroidism and treated primary hypothyroidism.
  
  
  METHODS
  Eighty-three subjects were included in this cross-sectional study: 36 with central hypothyroidism, 20 with primary hypothyroidism and 27 healthy controls. A clinical history and a physical examination, including height and weight measurement, were performed and body mass index (BMI) was calculated. Fasting blood was drawn to measure T3, T4, free T3, free T4 and TSH.
  
  
  RESULTS
  The free T3 to free T4 ratio was lower in treated central hypothyroidism than in euthyroidism but was similar to treated primary hypothyroidism. Free T4 was higher in treated central and primary hypothyroidism than in euthyroidism. Age, sex and BMI did not affect the free T3 to free T4 ratio.
  
  
  CONCLUSIONS
  Treated patients with central hypothyroidism had a lower free T3 to free T4 ratio, similar free T3 levels and higher free T4 concentrations than euthyroid controls, whereas all these parameters were similar in central and primary hypothyroid patients treated with T4. The question of whether these findings translate into adequate tissue concentrations of free thyroid hormones in all tissues remains to be answered. Further studies should aim to determine whether clinical outcomes could be improved by a treatment achieving more physiological plasma concentrations.
authors.affiliation: Servei d'Endocrinologia, Institut Universitari Dexeus, Barcelona, España. 30064gsl@comb.cat
authors.forename: Gemma
authors.initials: G
authors.lastname: Sesmilo
authors.forename: Olga
authors.initials: O
authors.lastname: Simó
authors.forename: Lucía
authors.initials: L
authors.lastname: Choque
authors.forename: Roser
authors.initials: R
authors.lastname: Casamitjana
authors.forename: Manel
authors.initials: M
authors.lastname: Puig-Domingo
authors.forename: Irene
authors.initials: I
authors.lastname: Halperin
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D005260:Female; D019382:Human Growth Hormone / Q000627:therapeutic use; D006801:Humans; D007018:Hypopituitarism / Q000150:complications / Q000188:drug therapy; D007037:Hypothyroidism / Q000097:blood* / Q000145:classification / Q000188:drug therapy / Q000209:etiology; D008297:Male; D008875:Middle Aged; D010911:Pituitary Neoplasms / Q000097:blood / Q000150:complications; D012737:Sex Factors; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*
chemical_list: D014284:Triiodothyronine; D019382:Human Growth Hormone; D013972:Thyrotropin; D013974:Thyroxine