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Secretion of Thyroid Hormones
In response to negative feedback mechanism, the thyroglobulin molecule is taken
up by the endocytotic vesicles of thyrocytes. Upon fusion of endocytotic vesicles
with lysosomes, T3 and T4 are liberated in plasma by enzymatic proteolysis of thyroglobulin. The T3 is more active form of thyroid hormones but it is present in small
quantities in human body [1, 5].
Regulation of Thyroid Hormones
The synthesis of thyroid hormones is regulated by negative feedback effect of circulating T3 and T4 on anterior pituitary. Low blood levels of thyroid hormones trigger
the release of TRH and TSH, while high levels of these hormones cause a ‘shut
down’ to the release of TRH and TSH. Plasma iodide concentration is also an
important factor in regulation of thyroid hormones. An increased iodine intake with
increased plasma iodide concentration will result in increased hormone production
and decreased TSH secretion and vice versa [1, 2].
Transport, Metabolism and Excretion of Thyroid Hormones
The thyroid hormones bind with three plasma proteins: a ɑ-globulin named as
thyroxine- binding globulin (TBG), transthyretin formerly known as thyroid binding
pre-albumin (TTR) and albu-min. The variations in concentration of TBG are more
important than other binding proteins, as thyroid hormones have greater affinity for
TBG [6]. The liver is major metabolising site for thyroid hormones where they
undergo de-iodination, decarboxylation, deamination, glucuronidation and sulfation. The metabolised products are excreted primarily in bile and partly in urine [1, 2].
Functions of Thyroid Hormones
Thyroid hormones are primarily involved in growth, development and regulation of
metabolism including basal metabolic rate, carbohydrates/lipid metabolism, thermogenesis and oxygen consumption [1–3, 7, 8].
Impaired Thyroid Functions
Thyroid anomalies are among the most common endocrine disorders and subclinical thyroid disease in middle-aged and elderly is quite prevalent.
Y. H. Khan et al.
Secretion of Thyroid Hormones
In response to negative feedback mechanism, the thyroglobulin molecule is taken
up by the endocytotic vesicles of thyrocytes. Upon fusion of endocytotic vesicles
with lysosomes, T3 and T4 are liberated in plasma by enzymatic proteolysis of thyroglobulin. The T3 is more active form of thyroid hormones but it is present in small
quantities in human body [1, 5].
Regulation of Thyroid Hormones
The synthesis of thyroid hormones is regulated by negative feedback effect of circulating T3 and T4 on anterior pituitary. Low blood levels of thyroid hormones trigger
the release of TRH and TSH, while high levels of these hormones cause a ‘shut
down’ to the release of TRH and TSH. Plasma iodide concentration is also an
important factor in regulation of thyroid hormones. An increased iodine intake with
increased plasma iodide concentration will result in increased hormone production
and decreased TSH secretion and vice versa [1, 2].
Transport, Metabolism and Excretion of Thyroid Hormones
The thyroid hormones bind with three plasma proteins: a ɑ-globulin named as
thyroxine- binding globulin (TBG), transthyretin formerly known as thyroid binding
pre-albumin (TTR) and albu-min. The variations in concentration of TBG are more
important than other binding proteins, as thyroid hormones have greater affinity for
TBG [6]. The liver is major metabolising site for thyroid hormones where they
undergo de-iodination, decarboxylation, deamination, glucuronidation and sulfation. The metabolised products are excreted primarily in bile and partly in urine [1, 2].
Functions of Thyroid Hormones
Thyroid hormones are primarily involved in growth, development and regulation of
metabolism including basal metabolic rate, carbohydrates/lipid metabolism, thermogenesis and oxygen consumption [1–3, 7, 8].
Impaired Thyroid Functions
Thyroid anomalies are among the most common endocrine disorders and subclinical thyroid disease in middle-aged and elderly is quite prevalent.
Y. H. Khan et al.