Chapter 19: Thyroid Gland

Thyroid Gland

• The thyroid gland has two primary functions: secreting thyroid hormones to maintain optimal metabolism and secreting calcitonin to regulate calcium levels.
• The thyroid gland is not essential for life, but its absence or hypofunction during fetal and neonatal life results in severe mental retardation and dwarfism.

Thyroid Hormone Synthesis

• Thyroglobulin, a glycoprotein, is synthesized in thyroid cells and secreted into the colloid.
• Thyroid peroxidase, a membrane-bound enzyme, oxidizes and reacts with iodide to form thyroid hormones within thyroglobulin.
• The synthesis process involves the formation of monoiodotyrosine (MIT), diiodotyrosine (DIT), and finally, triiodothyronine (T3) and thyroxine (T4).

Regulation of Thyroid Secretion

• Thyroid function is regulated primarily by pituitary TSH (thyroid-stimulating hormone), which is increased by hypothalamic TRH and inhibited by circulating free T4 and T3.

Effects of TSH on the Thyroid

• TSH stimulates thyroid function, increases iodide binding, and induces the synthesis and secretion of thyroid hormones.
• Long-term TSH treatment leads to thyroid enlargement (goiter).

Effects of Thyroid Hormones

• Thyroid hormones stimulate O2 consumption, affect growth and development, regulate lipid metabolism, increase carbohydrate absorption, and influence red blood cell function.
• T3 binds to thyroid receptors (TR) in nuclei, and T4 can also bind, but less avidly.

Thyroid Hormone Distribution

• The normal human thyroid secretes approximately 80 μg/day of T4, 4 μg/day of T3, and 2 μg/day of RT3.
• A portion of T4 is converted to T3 (27 μg/day) and RT3 (36 μg/day), and some is prepared for excretion as conjugates (17 μg/day).

Protein Binding

• Thyroid hormones are relatively lipophilic and bind to proteins in plasma and tissues, with T4 and T3 being physiologically active in their free forms.
• Albumin, transthyretin, and thyroxine-binding globulin (TBG) are the primary proteins that bind thyroid hormones.

Thyroid Gland

• The thyroid gland has two primary functions: secreting thyroid hormones to maintain optimal metabolism and secreting calcitonin to regulate calcium levels.
• The thyroid gland is not essential for life, but its absence or hypofunction during fetal and neonatal life results in severe mental retardation and dwarfism.

Thyroid Hormone Synthesis

• Thyroglobulin, a glycoprotein, is synthesized in thyroid cells and secreted into the colloid.
• Thyroid peroxidase, a membrane-bound enzyme, oxidizes and reacts with iodide to form thyroid hormones within thyroglobulin.
• The synthesis process involves the formation of monoiodotyrosine (MIT), diiodotyrosine (DIT), and finally, triiodothyronine (T3) and thyroxine (T4).

Regulation of Thyroid Secretion

• Thyroid function is regulated primarily by pituitary TSH (thyroid-stimulating hormone), which is increased by hypothalamic TRH and inhibited by circulating free T4 and T3.

Effects of TSH on the Thyroid

• TSH stimulates thyroid function, increases iodide binding, and induces the synthesis and secretion of thyroid hormones.
• Long-term TSH treatment leads to thyroid enlargement (goiter).

Effects of Thyroid Hormones

• Thyroid hormones stimulate O2 consumption, affect growth and development, regulate lipid metabolism, increase carbohydrate absorption, and influence red blood cell function.
• T3 binds to thyroid receptors (TR) in nuclei, and T4 can also bind, but less avidly.

Thyroid Hormone Distribution

• The normal human thyroid secretes approximately 80 μg/day of T4, 4 μg/day of T3, and 2 μg/day of RT3.
• A portion of T4 is converted to T3 (27 μg/day) and RT3 (36 μg/day), and some is prepared for excretion as conjugates (17 μg/day).

Protein Binding

• Thyroid hormones are relatively lipophilic and bind to proteins in plasma and tissues, with T4 and T3 being physiologically active in their free forms.
• Albumin, transthyretin, and thyroxine-binding globulin (TBG) are the primary proteins that bind thyroid hormones.