Abstract
The major product secreted by the thyroid is thyroxine (T4), whereas most of the biologically active triiodothyronine (T3) derives from the peripheral conversion of T4 into T3. The deiodinase enzymes are involved in activation and inactivation of thyroid hormones (THs). Type 1 and type 2 deiodinase (D1 and D2) convert T4 into T3 whereas D3 degrades T4 and T3 into inactive metabolites and is thus the major physiological TH inactivator. The hypothalamic-pituitary-thyroid axis maintains circulating TH levels constant, while the deiodinases tissue-specifically regulate intracellular thyroid status by controlling TH action in a precise spatio-temporal fashion. Here we review the data related to the recent identification of a paraneoplastic syndrome called “consumptive hypothyroidism,” which exemplifies how deiodinases alter substantially the concentration of TH in blood. This syndrome results from the aberrant uncontrolled expression of D3 that can induce a severe form of hypothyroidism by inactivating T4 and T3 in defined tumor tissue. This rare TH insufficiency generally affects patients in the first years of life, and has distinct features in terms of diagnosis, treatment, and prognosis with respect to other forms of hypothyroidism.
Highlights
Thyroid hormones (THs) are circulating hormones widely involved in the development and metabolic homeostasis of virtually all mammalian tissues
We review the data related to the recent identification of a paraneoplastic syndrome called “consumptive hypothyroidism,” which exemplifies how deiodinases alter substantially the concentration ofTH in blood.This syndrome results from the aberrant uncontrolled expression of D3 that can induce a severe form of hypothyroidism by inactivatingT4 andT3 in defined tumor tissue.This rareTH insufficiency generally affects patients in the first years of life, and has distinct features in terms of diagnosis, treatment, and prognosis with respect to other forms of hypothyroidism
We focus on D3 as the cause of consumptive hypothyroidism, a newly recognized clinical entity which often requires close cooperation between clinical endocrinologists, pediatricians, and oncologists
Summary
Thyroid hormones (THs) are circulating hormones widely involved in the development and metabolic homeostasis of virtually all mammalian tissues. TSH concentration was 177 mUI/ml, serum T4 concentration was 2.5 μg/dl, T3 concentration was low (15 ng/ml), and the level of rT3 was elevated (413 ng/ml) Based on these TH values, replacement hormone therapy was increased to 50 μg/day of LT4, and intravenous administration of 90 μg/day liothyronine was initiated. This therapy resulted in normalization of TSH and T3, but T4 levels remained low. This replacement dose of hormone therapy is very high considering that an athyreotic infant of the same age requires about 7–10 μg/kg/day of LT4 (equivalent to about 3 μg/kg/day of T3), which is about nine times lower than that required by this patient. An evaluation of all these cases strongly suggests that the development of consumptive hypothyroidism is related to the elevated D3 enzyme, which is in turn directly proportional to the size of the tumor
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