The Effects of Thyroid hormone (6)

 Thyroid hormone (triiodothyronine (T3) and thyroxine (T4)), produced by the thyroid gland, plays an important role in regulation of metabolism, including directly boosts energy metabolism and triggers rapid protein synthesis and regulates mitochondrial gene transcription, etc. Iodine is necessary for the production of T₃ and T₄, deficiency of Iodine can lead to enlarge thyroid grand and goitre.
 51. Thyroid hormone and cardiovascular functions
Thyroid hormone is well known for its direct effects in regulating cardiovascular function and metabolism. In the recent meta-analyses of population-based studies with long-term follow-up have clarified the risk of cardiovascular disorders in patients with subclinical thyroid dysfunction, researchers at the Inwendige Geneeskunde - Endocrinologie en Metabolisme, showed that treatment of patients with TSH levels between 0.1-0.4 mU/l or 4-10 mU/l should depend on other risk factors and patient age, with no treatment for persons with a TSH level of 4-10 mU/l who are older than 65 years(51).

52. Thyroid dysfunction are not associated with normocholesterolemia or hypercholesterolemia
In the study to assess in a clinically healthy, middle-aged population of employees the prevalence of thyroid function disorders and their relation to demographic variables and cardiovascular risk factors, found that the cardiovascular risk profile of subjects with mild subclinical hypothyroidism was not different from subjects with normal TSH levels. The prevalence of subclinical hypothyroidism was 0.8% in normocholesterolemic (cholesterol <5.2 mmol/l) and 1.4% in hypercholesterolemic subjects (n.s.). One woman each with the subclinical form of the disease developed hypothyroidism or hyperthyroidism after 21 and 11 months of follow-up, respectively. Subclinical hypothyroidism and subclinical hyperthyroidism were rarely observed in a target group for coronary heart disease prevention. Mild subclinical hypothyroidism was not associated with any adverse cardiovascular risk profile. These results argue against indiscriminate measurements of TSH concentrations in clinically healthy subjects either with normocholesterolemia or hypercholesterolemia(52).

53. Subclinical hypothyroidism treatment?
Subclinical thyroid dysfunction is characterized by normal levels of thyroid hormones but abnormal values of thyrotropin (TSH) in an asymptomatic individual. It is a common disorder with a prevalence of about 7 to 8% in women (most frequently in females over 50 years), and about 3% in men. In the study of Is there a need for treatment in subclinical hypo- and hyperthyroidism?, researchers indicated that most frequently, this disorder is caused by exogenous L-thyroxine treatment. The endogenous form of subclinical hyperthyroidism mainly caused by nodular goiter has a prevalence of up to 20% in patients with large goiters. In patients with subclinical hyperthyroidism, there is an increased risk for development of atrial fibrillation and for a decrease in bone mass in postmenopausal women. In the majority of patients measurable TSH levels can be detected before or after stimulation with TRH. This formally excludes overt hyperthyroidism in such patients. Frequently, there is no need for treatment but follow-up is important. However, in patients with subclinical hyperthyroidism associated with atrial fibrillation a therapy with antithyroid drugs, beta-blockers or radioiodine must be considered(53).

54. Thyroid function and postmenopause
The rate of thyroid cancer increases with age. The incidence of thyroid disease in a population of postmenopausal women is as follows: clinical thyroid disease, about 2.4%; subclinical thyroid disease, about 23.2%. Among the group with subclinical thyroid disease, 73.8% are hypothyroid and 26.2% are hyperthyroid. According to the Institute for Medical Research and Education (IMRE), Essen,  It is of importance that even mild thyroid failure can have a number of clinical effects such as depression, memory loss, cognitive impairment and a variety of neuromuscular complaints. Myocardial function has been found to be subtly impaired. There is also an increased cardiovascular risk, caused by increased serum total cholesterol and low-density lipoprotein cholesterol as well as reduced levels of high-density lipoprotein. These adverse effects can be improved or corrected by L-thyroxine replacement therapy(54).

55. Thyroid hormone levels and vegetative dysfunction and heart connective tissue dysplasia (HCTD)
Heart connective tissue dysplasia (HCTD) are known to be subject to infectious and inflammatory diseases due to peculiarities of their immune system. In the study of 181 patients with HCTD depending on thyroid hormone levels and vegetative dysfunction. indicated that HCTD was shown to be associated with a significant decrease of IgM levels and increase of circulating immune complexes. The IgG level in patients with mitral valve prolapse (MVP), anomalous chord localization, and combination of MPV and tricuspid valve prolapse was significantly higher than in the absence of HCTD(55).

56. Thyroid physiology and common diseases in pregnancy
Thyroid diseases are common during pregnancy and an adequate treatment is important to prevent adverse maternal and fetal outcomes, such as neurodevelopment complications in the fetus. According to the study by Artemisia Fetal-Maternal Medical Center, found that large-scale intervention trials are urgently needed to assess the efficacy of preconception or early pregnancy screening for thyroid disorders. Accurate interpretation of both antepartum and postpartum levels of thyroid hormones is important in preventing pregnancy-related complication secondary to thyroid dysfunction(56).

57. Subclinical hypothyroidism in pregnancy
In the study to evaluate the intellectual development of children of mothers who had M-SCH during the pregnancy for these children with 68 children were recruited, after excluding those age < 4 or age > 15, 44, found that IQ level and cognitive performance of children born to LT4-treated hypothyroid mothers is similar in those whose mothers have M-SCH during pregnancy compared with those whose mothers have normal serum TSH concentrations during pregnancy(57).

58. Thyroid disease in pregnancy and childhood
Thyroid function in pregnancy is characterised by a T4 surge at 12 weeks declining thereafter. Fetal brain development depends on T4 transport into the fetus which in turn depends on sufficient maternal iodine supply. In the study to measure free T4 using direct equilibrium dialysis, as well as total T4 and TSH in 287 pregnant women at 27 weeks' gestation, found that Increasing maternal TSH was related to better performance on tests of cognition and language at 12 months but not at later ages. At 60 months, there was inconsistent evidence that higher TSH was related to improved attention. We found no convincing evidence that maternal TH during the second half of pregnancy was related to impaired child neurodevelopment(58).

59. Hereditary medullary thyroid carcinoma (HMTC)
In the study to investigate the role of germline inheritance of polymorphisms in CYP1A2*F, CYP1A1m1, GSTP1, NAT2 and TP53 genes in hereditary medullary thyroid carcinoma (HMTC) patients of 132 patients with HMTC, 88 first-degree relatives of HMTC patients and 575 control individuals, found that
the inheritance of specific genes determining the individual response to environmental toxins may contribute to the risk and phenotypic variability that exists in patients with HMTC(59).

60. Thyroid Nodule Size and Cancer
In the study to evaluate the association of nodule size upon cancer risk in a retrospective cohort analysis at an academic hospital with 4955 consecutive patients evaluated between 1995 and 2009, found that Increasing thyroid nodule size impacts cancer risk in a nonlinear fashion. A threshold is detected at 2.0 cm, beyond which cancer risk is unchanged. However, the risk of follicular carcinomas and other rare thyroid malignancies increases as nodules enlarge(60).
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Sources
(51) http://www.ncbi.nlm.nih.gov/pubmed/23218037
(52) http://www.ncbi.nlm.nih.gov/pubmed/10880781
(53) http://www.ncbi.nlm.nih.gov/pubmed/10434773
(54) http://www.ncbi.nlm.nih.gov/pubmed/12724022
(55) http://www.ncbi.nlm.nih.gov/pubmed/23285764
(56) http://www.ncbi.nlm.nih.gov/pubmed/22419883
(57) http://www.ncbi.nlm.nih.gov/pubmed/21943136
(58) http://www.ncbi.nlm.nih.gov/pubmed/22132346
(59) http://www.ncbi.nlm.nih.gov/pubmed/23278115
(60) http://www.ncbi.nlm.nih.gov/pubmed/23275525