Thyroid disease is defined as a condition of malfunction of thyroid.
Hyperthyroidism is a condition in which the thyroid gland is over active
and produces too much thyroid hormones. Hypothyroidism is a condition
in which the thyroid gland is under active and produces very little
thyroid hormones. Thyroid cancer is defined as condition in which the
cells in the thyroid gland have become cancerous.
Thyroid hormone resistance syndrome
Thyroid hormone resistance syndrome is defined as a condition of which
affected individuals have elevated serum thyroid hormone levels and
inappropriately normal or elevated thyroid-stimulating hormone (TSH) but
are usually clinically euthyroid and require no treatment. Selective
pituitary resistance to thyroid hormone (PRTH) is characterized by
resistance in the pituitary gland but not in peripheral tissues(a).
B. The Causes and Risk Factors
B.1. Causes
Genetic mutation
Thyroid hormone resistance occurs when a genetic mutation in the thyroid
hormone receptor leads to reduced hormone binding affinity; the
concentration of free thyroid hormone in the circulation is inversely
correlated with the hormone binding affinity of the mutant receptor(6),
such as mutation in the THRbeta gene, A317T(7) and E333D(8). According
to the study of Syndromes of reduced sensitivity to thyroid hormone:
genetic defects in hormone receptors, cell transporters and deiodination
by the University of Chicago, Mutations in MCT8 and SECISBP2 have also
been associated with this condition(9) and a patient with the unusual
coincidence of two rare congenital disorders, lingual ectopy of the
thyroid gland and resistance to thyroid hormone (RTH), resulting in
impaired thyroid hormone production and action, respectively(10).
Thyroid hormone resistance syndrome affect approximately 1 in 40,000
live births and of of which over 100 different mutations have been
identified.
B.2. Risk factors
1. Dominant inheritance
Resistance to thyroid hormone (RTH) is an inherited syndrome of reduced
tissue responsiveness to thyroid hormone. To date, all individuals
expressing the RTH phenotype have been found to harbor mutations in the
thyroid hormone receptor beta (TR beta) gene that impair T3-mediated
function(11). Others reported a baby at age 70 days, an R243W mutation
in thyroid hormone receptor β was detected in our patient; while absent
in his mother, the mutation was present in his father, who never showed
any symptoms(12).
2. Graves’ disease
Resistance to thyroid hormone is a syndrome caused by thyroid hormone
receptor β mutations, which are usually inherited in an
autosomal-dominant pattern. There is a report of a resistance to
thyroid hormone in a neonate presenting with hyperthyroid symptoms born
to a mother with Graves’ disease and treated with methimazole and
iodine(12).
3. Minor alterations at the DNA level
Mutations in the thyroid hormone receptor (TR) beta gene are responsible
for RTH and 122 different mutations have now been identified belonging
to 300 families. With the exception of one family found to have complete
deletion of the TRbeta gene, all others have been demonstrated to have
minor alterations at the DNA level, according to the study by the Stoke
Mandeville Hospital(13).
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Sources
(a) http://www.ncbi.nlm.nih.gov/pubmed/8475937
(7) http://www.ncbi.nlm.nih.gov/pubmed/19227423
(8) http://www.ncbi.nlm.nih.gov/pubmed/17177139
(9) http://www.bprcem.com/article/S1521-690X%2807%2900026-7/abstract
(10) http://www.ncbi.nlm.nih.gov/pubmed/16053391
(11) http://www.ncbi.nlm.nih.gov/pubmed/8954015
(12) http://www.ncbi.nlm.nih.gov/pubmed/22905724
(13) http://www.ncbi.nlm.nih.gov/pubmed/17132274
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