Peripheral Thyroid Hormone Conversion and Its Impact on TSH and
Metabolic Activity
There have been recent advances in understanding of the local control of thyroid activity and metabolism, including deiodinase activity and thyroid hormone membrane transport. The goal of this review is to increase the understanding of the clinical relevance of cellular deiodinase activity. The physiologic significance of types 1, 2 and 3 deiodinase (D1, D2 and D3, respectively) on the intracellular production of T3 are discussed along with the importance and significance of the production of reverse T3. The difference in the pituitary and peripheral activity of these deidoidinases under a wide range of common physiologic conditions results
in different intracellular T3 levels in the pituitary and peripheral tissues, resulting in the inability to detect low tissue levels of thyroid hormone in peripheral tissues with TSH testing. This review demonstrates that extreme caution should be used in relying on TSH or serum thyroid levels to rule out hypothyroidism in the presence of a wide range of conditions, including physiologic and emotional stress, depression, dieting, obesity, leptin insulin resistance, diabetes, chronic fatigue syndrome, fibromyalgia, inflammation, autoimmune disease, or systemic illness, as TSH levels will often be normal despite the presence of significant hypothyroidism. The review discusses the significant clinical benefits of thyroid replacement in such conditions despite having normal TSH levels and the superiority of T3 replacement instead of standard T4 therapy.
Introduction
There have been recent advances in understanding of the local control of thyroid activity and metabolism, including deiodinase activity and thyroid hormone membrane transport. The goal of this review is to increase the understanding of the clinical relevance of cellular deiodinase activity. The physiologic significance of types 1, 2 and 3 deiodinase (D1, D2 and D3, respectively) on the intracellular production of T3 are discussed along with the importance and significance of the production of reverse T3. The difference in the pituitary and peripheral activity of these deidoidinases under a wide range of common physiologic conditions results in different intracellular T3 levels in the pituitary and peripheral tissues, resulting in the inability to detect low tissue levels of thyroid hormone in peripheral tissues with TSH testing. This review demonstrates that extreme caution should be used in relying on TSH or serum thyroid levels to rule out hypothyroidism in the presence of a wide range of conditions, including physiologic and emotional stress, depression, dieting, obesity, leptin insulin resistance, diabetes, chronic fatigue syndrome, fibromyalgia, inflammation, autoimmune disease, or systemic illness, as TSH levels will often be normal despite the presence of significant hypothyroidism. The review discusses the significant clinical benefits of thyroid replacement in such conditions despite having normal TSH levels and the superiority of T3 replacement instead of standard T4 therapy.
To accurately assess thyroid function, it must be understood that deiodinase enzymes are essential control points of cellular thyroid activity that determine intracellular activation and deactivation of thyroid hormones. This local control of cellular thyroid levels is mediated through three different deiodinase enzymes present in different tissues in the body; type I deiodinase (D1) and type II deiodinase (D2) increase cellular thyroid activity by converting inactive thyroxine (T4) to the active triiodothyronine (T3) while type III deiodinase (D3) reduces cellular thyroid activity by converting T4 to the anti-thyroid reverse T3 (reverse T3)1–9
The activity of each type of deiodinase enzyme changes in response to differing physiologic conditions, and this local control of intracellular T4 and T3 levels results in different tissue levels of T4 and T3 under different conditions. Because it is the activity of these deiodinases and transport of T4 and T3 into the cell that determines tissue and cellular thyroid levels and not serum thyroid levels, serum thyroid hormone levels may not necessarily predict tissue thyroid levels under a variety of physiologic conditions.
©2014, Kent Holtorf, MD
Journal Compilation ©2014, AARM
Journal of Restorative Medicine 2014;
DOI 10.14200/jrm.2014.3.0103
