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Questions and Answers
What are the two major iodine-containing thyroid hormones derived from tyrosine?
What are the two major iodine-containing thyroid hormones derived from tyrosine?
What hormone is responsible for the stimulation of the thyroid gland?
What hormone is responsible for the stimulation of the thyroid gland?
What is the significance of converting T4 to T3 in extra-thyroidal tissues?
What is the significance of converting T4 to T3 in extra-thyroidal tissues?
What is the primary role of the thyroid gland in the body?
What is the primary role of the thyroid gland in the body?
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What are the effects of thyroid hormones on the cardiovascular system?
What are the effects of thyroid hormones on the cardiovascular system?
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What is the primary mechanism of thyroid hormone action in target tissues?
What is the primary mechanism of thyroid hormone action in target tissues?
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Thyroid hormone deficiency in children can lead to stunted growth.
Thyroid hormone deficiency in children can lead to stunted growth.
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Which of these is NOT a cause of goiter?
Which of these is NOT a cause of goiter?
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What is the name of the condition caused by over-secretion of thyroid hormone?
What is the name of the condition caused by over-secretion of thyroid hormone?
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What is the proper term for the thyroid gland's storage site for thyroid hormone?
What is the proper term for the thyroid gland's storage site for thyroid hormone?
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What is the role of the NIS (Na-Iodine-Symporter) in thyroid hormone synthesis?
What is the role of the NIS (Na-Iodine-Symporter) in thyroid hormone synthesis?
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Which protein is responsible for the attachment of iodine to tyrosine residues within the thyroglobulin molecule?
Which protein is responsible for the attachment of iodine to tyrosine residues within the thyroglobulin molecule?
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Where does the conversion of T4 to T3 primarily occur?
Where does the conversion of T4 to T3 primarily occur?
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What is the primary role of growth hormone in the body?
What is the primary role of growth hormone in the body?
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What is the role of GHRH in growth hormone regulation?
What is the role of GHRH in growth hormone regulation?
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What is the role of somatostatin in growth hormone regulation?
What is the role of somatostatin in growth hormone regulation?
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What are the primary target tissues for insulin-like growth factors (IGFs)?
What are the primary target tissues for insulin-like growth factors (IGFs)?
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What is the primary mechanism of action for IGF-1?
What is the primary mechanism of action for IGF-1?
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What is the term used to describe the enlargement of the bones of the hands, feet, and face, often associated with acromegaly?
What is the term used to describe the enlargement of the bones of the hands, feet, and face, often associated with acromegaly?
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Which of the following factors contributes to the regulation of growth?
Which of the following factors contributes to the regulation of growth?
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Study Notes
Endocrine: Thyroid and Growth Axes and Integration across Axes
- The thyroid is a lobed gland located below the larynx that regulates metabolism.
- The thyroid gland produces the hormones T3 and T4, which are derived from tyrosine and contain iodine.
- Learning Objectives for the Thyroid include identifying the biosynthesis, storage, and secretion of T3 and T4, and their regulation; describing the absorption, uptake, distribution, and excretion of iodide; explaining the importance of thyroid hormone binding in blood on free and total thyroid hormone levels.
- The significance of converting T4 to T3 and reverse T3 (rT3) in extra-thyroidal tissues needs to be understood; as well as the physiologic effects and mechanisms of action of thyroid hormones.
- The causes and consequences of over-secretion and under-secretion of thyroid hormones need to be understood, along with what conditions can cause an enlargement of the thyroid gland.
Learning Objectives – Thyroid
- Identify the processes of triiodothyronine (T3) and thyroxine (T4) biosynthesis, storage, and secretion, and their regulation.
- Describe the absorption, uptake, distribution, and excretion of iodide.
- Explain the importance of thyroid hormone binding in the blood, concerning free and total thyroid hormone levels.
- Understand the significance of the conversion of T4 to T3 and reverse T3 (rT3) in extra-thyroidal tissues.
- Describe the physiological effects and mechanisms of action of thyroid hormones.
- Understand the causes and consequences of thyroid hormone over-secretion and under-secretion.
- Explain conditions that cause thyroid gland enlargement.
The Thyroid is a Lobed Gland Below the Larynx that Regulates Metabolism
- The hypothalamus releases TRH (Thyrotropin-releasing hormone).
- The anterior pituitary releases TSH (Thyroid-stimulating hormone) in response to TRH.
- The thyroid produces thyroid hormones (T3 and T4).
- Thyroid hormones increase metabolic rate and heat production, enhance growth and CNS development, and increase sympathetic activity.
Signal Transduction Pathways Used by Major Hormones
- Cyclic AMP/PKA pathway: Alters preexisting proteins.
- IP3/Ca2+ and DAG/PKC pathway: Alters preexisting proteins.
- Tyrosine kinase pathway: Alters preexisting proteins, turns on gene transcription, leading to synthesis of designated protein.
- JAK/STAT pathway: Turns on gene transcription, leading to synthesis of designated protein.
- Hormone-response elements on DNA: Turns genes on, leading to synthesis of designated proteins.
Synthesis, Storage, and Secretion of Thyroid Hormone into Circulation
- Tyrosine-containing thyroglobulin (Tg) is produced in follicular cells and transported to the colloid.
- Iodide is actively transported into the colloid.
- Iodide is oxidised to an active form by thyroperoxidase (TPO) at the luminal membrane.
- Active iodide exits the cell at the lumina channels.
- Iodination of tyrosine produces monoiodotyrosine (MIT) and diiodotyrosine (DIT).
- Coupling MIT with DIT produces T3.
- Coupling DIT with DIT produces T4.
- Thyroid follicular cells engulf the Tg-containing colloid by phagocytosis.
- Lysosomes in the follicular cells break down the Tg, releasing T3 and T4 into the blood.
Learning Objectives – Thyroid
- Identify the steps in the biosynthesis, storage, and secretion of triiodothyronine (T3) and thyroxine (T4) and their regulation.
- Describe the absorption, uptake, distribution, and excretion of iodide.
- Explain the importance of thyroid hormone binding in blood on free and total thyroid hormone levels.
- Understand the significance of the conversion of T4 to T3 and reverse T3 (rT3) in extra-thyroid tissues.
- Describe the physiologic effects and mechanisms of action of thyroid hormones.
- Understand the causes and consequences of a) over-secretion and b) under-secretion of thyroid hormones.
- Explain conditions that cause thyroid gland enlargement.
Factors Regulating Growth
- Genetics
- Maternal/fetal/placental factors (not including GH).
- Appropriate levels of growth-influencing hormones (GHRH).
- Adequate diet with sufficient protein and essential amino acids.
- Avoid chronic diseases and environmental stress.
Growth Hormones Regulation
- Growth hormone secretion is regulated by hypothalamic hormones (GH releasing hormone and somatostatin), glucose levels, and insulin-like growth factor 1 (IGF-1).
- The target organs or cells affected by insulin-like growth factors in longitudinal growth are detailed.
- The metabolic and growth-promoting actions of growth hormone are described, including its impact on cell division, protein synthesis, and bone growth.
- The role of somatostatin in inhibiting growth hormone and thyroid-stimulating hormone (TSH) to control growth and metabolism is explained.
- Cell signaling defects that cause specific endocrine diseases in the thyroid and growth axes are detailed.
Causes of Goiter Formation
- Iodine deficiency
- Autoimmune diseases (e.g., Graves' disease)
- Eating large amounts of soy, brassica vegetables, and peanuts (which contain antithyroid factors).
- Smoking (compounds in cigarettes can decrease the uptake of iodine).
HPT Axis Dysfunction
- Causes and symptoms of hyperthyroidism and hypothyroidism are described in a table.
- Different hormone levels are discussed, including T3, T4, and TSH.
Thyroid Storm
- A massive release of T4 and increased sensitivity to catecholamines can elevate cardiac muscle's metabolic demand, potentially causing myocardial ischemia.
- Symptoms include high fever, agitation, delirium, congestive heart failure, and loss of consciousness.
Hashimoto's Disease
- Hashimoto's disease is a type of hypothyroidism.
- Autoimmune factors damage the thyroid gland.
- Common symptoms include weight gain, cold intolerance, fatigue, slow pulse, slow reflexes, depression, and memory problems.
Euthyroid Sick
- Patients may exhibit hypothyroid symptoms but have underlying illnesses affecting thyroid hormone conversion and rT3 levels.
- Diagnosis depends on the ratio of T3 to rT3.
T3 and T4 are Important Regulators of Basal Metabolic Rate
- Their actions increase the basal metabolic rate, which influences oxygen consumption and energy expenditure.
- They play a key role in calorigenic reactions and the synthesis and degradation of carbohydrates, fats, and proteins.
- Thyroid hormones affect the body by stimulating metabolic processes.
Sympathomimetic Effects of Thyroid Hormone
- Thyroid hormones affect the body in a manner similar to the sympathetic nervous system (SNS).
- Target cells become more responsive to epinephrine and norepinephrine.
- Thyroid hormones increase the number of catecholamine receptors on target cells.
- The thyroid hormones enable the body to handle stress.
Growth Spurts During Childhood
- Postnatal growth spurts are not gender-dependent.
- Pubertal growth spurts are influenced by sex hormones (androgens and estrogens).
- Timing of these growth spurts varies between genders.
Important Note about the Growth Hormone Axis
- The Hypothalamic-Pituitary-Growth hormone (HPG) axis, regulates growth hormone secretion by receiving inputs from the hypothalamus and anterior pituitary.
Growth Hormone Functions
- The growth hormone has direct and indirect effects on metabolism and stimulates growth, through cell division and protein synthesis and bone growth.
IGF Signaling
- Insulin-like growth factors (IGFs) signal via Receptor Tyrosine Kinases, altering preexisting protein, activating protein kinase receptors.
- The pathway leads to increased and controlled cell growth and proliferation, crucial for various functions.
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Description
Explore the thyroid gland's function in metabolism and hormones T3 and T4. Understand their biosynthesis, secretion, and the significance of iodide in hormone regulation. This quiz covers thyroid hormone effects, conversion, and conditions related to secretion levels.
