title: Increased thyroidal T4 to T3 conversion in autonomously functioning thyroid adenoma: from euthyroidism to thyrotoxicosis.
abstract: OBJECTIVE
  The aim was to investigate whether the intrathyroid conversion of T4 to T3 in autonomously functioning thyroid adenoma (AFTA) tissue could influence serum T3 levels and suppression of TSH, especially in patients with borderline thyroid function.
  
  
  METHODS
  In ten patients with AFTA, thyroidal conversion of T4 to T3 was investigated in nodular and paranodular, TSH-suppressed tissue. All patients had normal serum T4 and suppressed TSH. Serum T3 was normal in six, and borderline or slightly increased in four. AFTA and paranodular tissues were surgically removed and frozen at -70°C, then homogenized in a glass homogenizer, centrifuged at 100,000×g, and particulate fraction collected as a pellet. Analysis mixture consisted of thyroid enzyme suspension in 50 μmol/L TRIS buffer with 5 μmol DTT and 200 μL 1.3 μmol T4. Incubation was performed at 37°C and the generation of T3 measured after 5, 10, 20 and 40 minutes respectively.
  
  
  RESULTS
  T3 production (pmol/mg protein) was significantly higher in AFTA than in paranodular tissues (8.8 1.2/Mean ± SE/vs. 1.8 ± 0.2; p<0.01), and excessively high (9.8, 14.1, 14.2 and 15.0) in four patients with borderline or slightly supranormal serum T3. A significant correlation was found between serum T3 concentrations and T3 generation (T4 conversion) in AFTA tissues.
  
  
  CONCLUSIONS
  Results suggest that increased thyroidal T4 to T3 conversion in AFTA tissue could be involved in an increased delivery of T3, increased serum T3 and suppressed serum TSH, particularly in patients with the disease evolving from euthyroid to an early hyperthyroid phase.
authors.affiliation: Department of Endocrinology, Sisters of Mercy University Hospital Center, University of Zagreb, School of Medicine, Vinogradska , Croatia. miljenkosolter@yahoo.com
authors.forename: M
authors.initials: M
authors.lastname: Solter
authors.forename: L
authors.initials: L
authors.lastname: Posavec
authors.forename: D
authors.initials: D
authors.lastname: Solter
authors.forename: V
authors.initials: V
authors.lastname: Vargek-Solter
mesh_terms: D000236:Adenoma / Q000378:metabolism* / Q000601:surgery; D000328:Adult; D006801:Humans; D008875:Middle Aged; D013961:Thyroid Gland / Q000378:metabolism*; D013964:Thyroid Neoplasms / Q000378:metabolism* / Q000601:surgery; D013971:Thyrotoxicosis / Q000378:metabolism*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood / Q000378:metabolism*; D014284:Triiodothyronine / Q000096:biosynthesis* / Q000097:blood
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine