Thyroid T4 5′-deiodinase activity in normal and abnormal human thyroid glands

Abstract

To investigate the activity of the thyroid gland to convert T4 to T3, we measured the activity of thyroid T4 5′-deiodinase in the following human thyroid glands: 9 normal glands, 5 Hashimoto's thyroiditis, 13 follicular adenomas, 11 methimazole (MMI)-treated Graves' disease (GD), 11 propranolol iodide-treated GD, and 8 propylthiouracil (PTU)-treated GD. The enzyme activity was determined by the ability of 100,000 × g pellet of the thyroid homogenate to convert T4 to T3 in vitro. Normal thyroids showed the enzyme activity of 1.59 ± 0.18 (mean ± SEM) pmol T3/mg protein/min. Euthyroid Hashimoto's thyroiditis displayed the enzyme activity of 1.01 ± 0.15 pmol T3/mg protein/min, which was similar to the normal thyroid enzyme activity. The hypothyroid gland of Hashimoto's thyroiditis showed the enzyme activity of 1.8 pmol T3/mg protein/min. Follicular adenomas showed a wide range of enzyme activity with the mean level of 3.24 ± 0.82 pmol T3/mg protein/min that did not differ significantly from that of the normal thyroids. Interestingly, one adenoma, despite TSH suppression that ordinarily decreases enzyme activity, showed the greatest activity of 11.0 pmol T3/mg protein/min. Graves' thyroids following treatment with MMI, PTU, and propranolol-iodide showed enzyme activities of 4.61 ± 0.53, 3.95 ± 0.43, and 3.51 ± 0.46 pmol T3/mg protein/min, respectively; all these values were greater than that of the normal thyroids ( P < 0.01), but did not differ significantly when compared with each other. In summary, thyroid glands with Hashimoto's thyroiditis had activities of T4 to T3 conversion similar to the normal thyroid glands. Variable degrees of T4 to T3 conversion can be expected in follicular adenoma tissue. In GD thyroid T4 5′-deiodinase activity was increased despite preoperative treatment with the enzyme inhibitors PTU or propranolol.