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Thyroid Hormone

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Thyroid
"Shaped like a shield"
Very important in early development, particularly neural
Thyrocytes
Rim of cells that form Thyroid Follicles
Make, secrete and store thyroid hormone
Thyroglobulin
Precursor to thyroid hormones (T3, T4)
Thyroglobulin Molecular Structure
Tyrosine (amino acid) + Iodine
Tyrosine
Amino acid
Non-essential: can be made from phenol alanine
Tyrosine Molecular Structure
Diiodotyrosine can be made by adding 2 I on each side of OH group
Phenol Alanine Hydroxylase
Makes tyrosine from phenol alanine
Sodium Potassium Symport
◦Symports move particles in the same direction
◦Responsible for moving iodide to thyrocyte
Thyroid Binding Globulin (TBG)
Carrier protein that transports thyroid hormone
Thyroid Hormone (T4) Molecular Structure
SEE EXAM #1 STUDY GUIDE
T4 (Tetraiodothyronine)
Half life: 15 days
Deiodinase
Process by which T4 becomes T3
T4 Effects
Mitochondria proliferates in presence of hormone and mitochondrial hypertrophy (enlargement) occurs (individual mitochondrion become bigger)
Thyrotropin/Thyroid-Stimulating Hormone (TSH)
"Thyroid growth"
Hormone from pituitary gland that controls thyrocytes
What Controls Thyrocytes?
TSH
Hypertrophy
Enlargement
Hyperplasia
Proliferation
Mitogenic
Encouragement of mitosis
Atrophy
Get smaller
How Does TSH Control Thyrocytes?
◦Hypertrophy = enlarges thyrocytes, allows it to make and secrete more T4
◦Hyperplasia = increase in number of cells, thyrocytes proliferate, mitogenic
What Controls the Pituitary Gland?
TRH
Thyrotropin-Releasing Hormone (TRH)
Neurohormone from small gland attached to the base of the brain that regulates pituitary gland by stimulating it to secrete TSH
What Controls the Brain in the Secretion of TRH?
Secretion of T4 reduces number of TRH receptors on
pituitary gland (fewer receptors, lower pituitary response to TRH)
• Down-Regulation of receptors
Goiter
Hypertrophied thyroid
Graves Disease
◦Type of hyperthyroidism
◦Autoimmune
◦Thyrocytes poisoned by abnormal antibodies (IgG) which compete with TSH for the same protein receptor and functions like TSH only more excessively
◦Increase in metabolic and heart rate (palpitations, arrhythmia), and oxygen consumption, exophthalmos, transplacental
◦High T4 levels will cause positive feedback on the brain, causing it to produce lower levels of TRH and therefore lower TSH
•Treatment 1: Radioactive iodine
•Treatment 2: Immunosuppression (Higher mortality rate)
•Treatment 3: Synthroid
•Treatment 4: Subtotal Thyroidectomy
•Treatment 5: Thiouracil
•Treatment 6: Thiocyanates
Thiocyanates
Found naturally in cabbage, inhibit sodium iodide transport
Symport
A membrane transport process that carries two substances in the same direction across the membrane.
Sodium Iodide Symports
Mechanism by which thyrocytes to take up iodine
Thiouracil
Inhibits thyroperoxidase which contributes to synthesis of thyroid hormone
Subtotal Thyroidectomy
Partial removal of the thyroid
Transplacental
Will be passed from mother to child
Radioactive Iodine Treatment
"Hot" or radioactive iodine will compete with the "cold" or regular iodine and cell that intakes radioactive iodine will be destroyed by the radiation
◦Sodium iodide symports can get thyrocytes to take up
radioactive iodine
◦Iatogenic: patient will emit gamma radiation and will be unsafe for a while
Thyroid Stimulating Immunoglobin (IgG)
Abnormal antibody that competes with TSH for the same protein receptor and functions like TSH only more excessively, causing Graves Disease
Exophthalmos
Eyes bulge because of eyeball volume increase so that
eyeball cannot fit in orbital cavity which will cause deteriorating vision
Carcinoma
Epithelial cancer
Adenocarcinoma
Secretory epithelial cancer
Hypothyroidism
Condition of hyposecretion of the thyroid gland causing low thyroid levels in the blood that result in sluggishness, slow pulse, and often obesity
Hyperthyroidism
Condition of hypersecretion of the thyroid gland characterized by exophthalmia, tachycardia, goiter, and tumor
Hashimoto's Thyroiditis
7x greater incidence than Graves disease
‣ Primary and Secondary Diagnosis
◦Autoimmune
◦Symptoms: Puffy eyes, apparent obesity (redistribution of polysaccharide), lethargy and stupor, low consumption of oxygen, decrease in metabolic rate
‣ Hashimoto antibody destroys TSH receptors, causes atrophy in thyroid > Insufficient T4 levels (brain is un-suppressed) elevation of both TRH and TSH
◦Treatment: Synthroid
Synthroid Treatment
Treatment with T4 when thyroid destroyed
Used to treat Hashimoto's Thyroiditis
Primary Diagnosis
Trouble found directly in thyroid
Secondary Diagnosis
Trouble found elsewhere, thyroid not directly responsible
Endemic
Characteristic to a region
Endemic Goiter
-Mostly found in inland populations
-Cause: dietary deficiency of iodine
‣ Adequate T4 cannot be made > brain un-suppressed by loss of T4 > elevated TRH > elevated TSH
• Still binds to TSH receptors and will cause hypertrophy and induce mitosis in thyrocytes
◦Treatment: iodine
Relationship Between TRH, TSH and T4
TRH > +TSH > +T4
•T4 will then act in positive feedback to inhibit TRH and TSH
Iatogenic
A disease that results from the adverse affects of treatment or surgery

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