96
level of leptin hormones in obesity results in increased secretion of TRH and TSH
and ultimate increase in fT3. Hyperthyroidism results in increased intestinal glucose absorption, hyperglycemia, elevated hepatic glucose turnover, raised levels of
fasting and postprandial insulin and pro-insulin leading to hyperinsulinemia and
marked rise in free fatty acid concentrations. While hypothyroidism is associated
with decreased intestinal absorption, decreased hepatic output and decreased
peripheral utilization of glucose. However, poor glycaemic control in diabetic
patients impairs the conversion of T3 to T4, decreases the TSH level and response
to TRH, and ultimate thyroid enlargement and development of thyroid nodules. In
hypothyroid patients, an increase in total cholesterol and LDL concentration, a
decrease in catabolism of LDL, decreased activity of LPL and ultimate decrease in
clearance of triglycerides rich lipoproteins are observed. An increase in activity of
catabolic pathways in hyperthyroid patients may lead to enhanced synthesis and
degradation of lipids, and hypertriglyceridemia. Hyperthyroidism may stimulate
osteoclasts formation resulting in reduced bone mineral density and increased risk
of fractures. However, the underlying mechanism involved in detrimental effects of
hypothyroidism on bone mineral density is still not clear.
Introduction to Thyroid Hormones (TH)
The nervous system controls the cellular and intracellular functions of body through
endocrine system. The thyroid gland is a major component of endocrine system and
regulates the metabolism of cells and tissues by producing thyroid hormones. The
two most important thyroid hormones are tri-iodothyronine (T3) and thyroxine
(T4) [1–3].
Biosynthesis of Thyroid Hormones
In response to certain stimuli, the hypothalamic thyrotropin-releasing hormone
(TRH) governs the release of thyroid stimulating hormone (TSH) from anterior
pituitary. The action of TSH on membrane receptors of thyroid cell follicle results
in the synthesis of thyroid hormones. This process is mediated by activation of
cAMP (cyclic adenosine monophosphate) and phosphatidyl inositol 3-kinases
(PI3Ks). Following steps are involved in the synthesis, storage and secretion of
thyroid hormones.
• Plasma iodide uptake by thyroid follicle cells
• Organification of iodide (Oxidation of iodide and incorporation in thyroglobulin)
• Iodination of tyrosine residues
• Secretion of thyroid hormones through endocytotic vesicles
Y. H. Khan et al.
level of leptin hormones in obesity results in increased secretion of TRH and TSH
and ultimate increase in fT3. Hyperthyroidism results in increased intestinal glucose absorption, hyperglycemia, elevated hepatic glucose turnover, raised levels of
fasting and postprandial insulin and pro-insulin leading to hyperinsulinemia and
marked rise in free fatty acid concentrations. While hypothyroidism is associated
with decreased intestinal absorption, decreased hepatic output and decreased
peripheral utilization of glucose. However, poor glycaemic control in diabetic
patients impairs the conversion of T3 to T4, decreases the TSH level and response
to TRH, and ultimate thyroid enlargement and development of thyroid nodules. In
hypothyroid patients, an increase in total cholesterol and LDL concentration, a
decrease in catabolism of LDL, decreased activity of LPL and ultimate decrease in
clearance of triglycerides rich lipoproteins are observed. An increase in activity of
catabolic pathways in hyperthyroid patients may lead to enhanced synthesis and
degradation of lipids, and hypertriglyceridemia. Hyperthyroidism may stimulate
osteoclasts formation resulting in reduced bone mineral density and increased risk
of fractures. However, the underlying mechanism involved in detrimental effects of
hypothyroidism on bone mineral density is still not clear.
Introduction to Thyroid Hormones (TH)
The nervous system controls the cellular and intracellular functions of body through
endocrine system. The thyroid gland is a major component of endocrine system and
regulates the metabolism of cells and tissues by producing thyroid hormones. The
two most important thyroid hormones are tri-iodothyronine (T3) and thyroxine
(T4) [1–3].
Biosynthesis of Thyroid Hormones
In response to certain stimuli, the hypothalamic thyrotropin-releasing hormone
(TRH) governs the release of thyroid stimulating hormone (TSH) from anterior
pituitary. The action of TSH on membrane receptors of thyroid cell follicle results
in the synthesis of thyroid hormones. This process is mediated by activation of
cAMP (cyclic adenosine monophosphate) and phosphatidyl inositol 3-kinases
(PI3Ks). Following steps are involved in the synthesis, storage and secretion of
thyroid hormones.
• Plasma iodide uptake by thyroid follicle cells
• Organification of iodide (Oxidation of iodide and incorporation in thyroglobulin)
• Iodination of tyrosine residues
• Secretion of thyroid hormones through endocytotic vesicles
Y. H. Khan et al.