Increased thyroidal T4 to T3 conversion in autonomously functioning thyroid adenoma: from euthyroidism to thyrotoxicosis.

AIM: 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. PATIENTS AND 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. CONCLUSION: 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.

Exogenous and endogenous suppression of thyroid-stimulating hormone induces similar effects on thyroidal iodothyronines.

Thyroidal concentrations of T4 and T3 and the T4/T3 ratio were analyzed in the nodular and paranodular tissues from two groups of patients with suppressed TSH secretion. The first group consisted of 17 patients with nontoxic nodular goitre (NG), 8 of whom received long-term levothyroxine therapy to suppress TSH, while remaining 9 were untreated. The second group consisted of 10 patients with autonomously functioning thyroid adenoma (AFTA), in whom TSH secretion was suppressed due to the adenoma-induced increase in thyroid hormone concentrations. In nodular tissues of NG patients, thyroidal T4 and the T4/T3 ratio were significantly higher in treated than in untreated patients (0.34 +/- 0.05 vs. 0.15 +/- 0.02 mol T4/mol of thyroglobulin (Tg) and 10.9 +/- 1.2 vs. 5.2 +/- 0.7 respectively). Analysis of paranodular tissues of NG patients also revealed a higher T4/T3 ratio in treated patients (16.0 +/- 2.1 vs. 6.9 +/- 0.9), although thyroidal T3 and T4 concentrations in treated and untreated patients were similar. In AFTA patients, both T3 and T4 concentrations were higher in the adenoma than in paranodular tissues (0.14 +/- 0.04 vs. 0.02 +/- 0.005 mol T3/mol Tg and 1.08 +/- 0.32 vs. 0.26 +/- 0.06 mol T4/mol Tg), whereas the T4/T3 ratio was significantly higher in paranodular tissues (23.2 +/- 5.9 vs. 9.3 +/- 1.8). These results indicate that suppression of TSH induced either exogenously or endogenously results in an increase in the thyroidal T4/T3 ratio that reflects an increase in T4 and/or a decrease in T3 concentrations. These findings also support the notion that TSH preferentially stimulates thyroidal T3 production.

Thyroid hormone economy in response to extreme cold exposure in healthy factory workers.

The effects of cold exposure on serum total T4 (TT4), total T3 (TT3), free T4 (FT4), free T3 (FT3), rT3, TSH, T4-binding globulin (TBG), and T3 resin uptake were investigated in 82 euthyroid factory workers. Twenty-five workers (group 1) were exposed intermittently (approximately 3.5 h daily) to extreme cold (-40 to -20 C) during the 8-h work shift, and 47 (group 2) were exposed to moderate cold (-10 to 8 C) for the entire 8 h. Ten individuals working at room temperature for the same period also were studied. After cold exposure, serum TT4 decreased in group 1 and did not change in group 2, whereas FT4 did not change in group 1 and increased in group 2. After exposure, serum TT3 and rT3 decreased significantly in both groups, while FT3 did not change in either. The basal serum TT4 levels in groups 1 and 2 were significantly lower than those in the control group, whereas those of FT4 and FT3 were higher. Thus, cold exposure had opposite effects on total thyroid hormones and their free fractions, consistent with a cold-induced decrease in thyroid hormone-binding capacity. A postexposure decrease in serum TBG was found in women in group 2, but not in men in either group 2 or group 1, suggesting that factors other than decreased TBG are also involved. The results suggest the possibilities that 1) decreased thyroid hormone-binding capacity is an adaptive response to cold exposure, and/or 2) increased free thyroid hormone levels in response to cold exposure result in a new higher equilibrium between extracellular and intracellular FT4 and FT3.