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

Area of Science:

• Endocrinology
• Molecular Biology
• Thyroid Research

Background:

• Thyroid hormone metabolism involves the conversion of thyroxine (T4) to the active form, triiodothyronine (T3).
• Thyroid T4 5 extquotesingle-deiodinase is the key enzyme responsible for this conversion.
• Understanding enzyme activity in various thyroid conditions is crucial for diagnosing and managing thyroid disorders.

Purpose of the Study:

• To investigate and compare the activity of thyroid T4 5 extquotesingle-deiodinase in normal human thyroid glands and those affected by Hashimoto's thyroiditis, follicular adenomas, and treated Graves' disease.
• To assess the impact of different treatments for Graves' disease on thyroid enzyme activity.

Main Methods:

• Human thyroid tissue samples were collected, including normal glands, Hashimoto's thyroiditis, follicular adenomas, and Graves' disease treated with methimazole (MMI), propranolol-iodide, or propylthiouracil (PTU).
• Thyroid T4 5 extquotesingle-deiodinase activity was measured in vitro by assessing the ability of thyroid homogenate to convert T4 to T3.
• Enzyme activity was quantified as pmol T3/mg protein/min.

Main Results:

• Normal thyroid glands exhibited an enzyme activity of 1.59 +/- 0.18 pmol T3/mg protein/min.
• Euthyroid Hashimoto's thyroiditis showed similar enzyme activity (1.01 +/- 0.15 pmol T3/mg protein/min) to normal glands; hypothyroid Hashimoto's showed 1.8 pmol T3/mg protein/min.
• Follicular adenomas displayed a wide range of activity (mean 3.24 +/- 0.82 pmol T3/mg protein/min), with one case showing exceptionally high activity.
• Graves' disease tissues treated with MMI, PTU, or propranolol-iodide demonstrated significantly elevated enzyme activities (4.61 +/- 0.53, 3.95 +/- 0.43, and 3.51 +/- 0.46 pmol T3/mg protein/min, respectively) compared to normal thyroids (P < 0.01).

Conclusions:

• Thyroid glands affected by Hashimoto's thyroiditis maintain T4 to T3 conversion activity comparable to normal thyroid glands.
• Treatments for Graves' disease, including MMI, PTU, and propranolol-iodide, lead to a significant increase in thyroid T4 5 extquotesingle-deiodinase activity.
• These findings highlight differential regulation of thyroid hormone conversion in various thyroid pathologies and treatment responses.