Follicles and Colloid

–Follicular Epithelial Cells

–Colloid- stores thyroid hormone

Parafollicular Cells(C cells) Follicles and Colloid • Calcitonin

Thyroglobulin • Iodinated tyrosine molecules • 1) Synthesis of Thyroglobulin – Follicular epithelial cells • Tetraiodothyronine = T4 – Tyrosine =Thyroxine – Secreted into the Colloid • Triiodothyronine = T 3 • 2) Iodide Trapping • Reverse T3 = rT3 = an isomer – Plasma into Epithelial Cells – Thiocyanates can block

1 h

• 3) Oxidation of Iodide • 4) Organification – MIT(monoiodothyronine) – Thyroid peroxidase – DIT(diiodothyronine) • oxidizes Iodide to form Iodine TPO • DIT + MIT = T3(10%) rT3 (<1%) I¯ Ð I • DIT + DIT = T4(90%) – Thiouracils can block

Secretion of Thyroid Hormones

• 5) STORAGE • T4 and T3 released Thryoglobulin from cell into plasma

• Tyrosine and I are recycled

Concept of Bound vs. Unbound • Thyroxine binding globulin (75% T-4; 75% T-3) • Concepts • Transthyretin (15% T-4; 0% T-3) • Thyroxine binding albumin • Clinical Implications (10% T-4; 25% T-3)

2 Thyroid Stimulating Hormone

• Enhances Iodide trapping * • Enhances synthesis of T4 and T3 • Enhances the endocytosis of the colloid • Enhances the proteolysis of the thyroglobulin • Enhances the release of T4 and T3 * Starvation inhibits this enzyme

• Maturational/Differentiational Mechanism of Action • Neurological Effects

•Growth

• Metabolic Actions Hypothyroidism • Primary - failure of gland itself • Sympathomimetic Effect • Secondary - TRH or TSH related deficiency • Reproduction • Nutritional (most common world) • Hashimoto thyroiditus most common U.S.

3 • Symptoms (Infants and • Symptoms (Adults) Children) – Low BMR – R.D.S. – Poor resistance to cold – Retarded Bone Age – Weight gain – Mental Retardation – Slow thinking – Stunted Growth – Thin and brittle hair – Delayed or absent sexual – Weakness, fatigue, maturity lethargy – MYXEDEMA

Hypothyroidism Hyperthyroidism (Thyrotoxicosis)

Treatment • Graves’ Disease – THYROXINE PREPARATIONS • Symptoms – Increased BMR – Heat Intolerance • LEVOXYL – Increased appetite – Elevated HR and • SYNTHROID arrhythmias, – Nervous, irritable • LEVOTHYROID – Exopthalmus

Hyperthyroidism • Treatment – Surgery – 131I – Thiouracils – (PROPACIL)

4 Goiter

• Enlarged Thyroid Gland • Hyperthyroidism or Hypothyroidism • Iodide deficiency

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