Thyroid Hormones Overview
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Questions and Answers

What percentage of thyroid hormones is bound to thyroxine-binding globulin (TBG)?

  • 50%-60%
  • 90%-99%
  • 70%-80% (correct)
  • 80%-90%

Which of the following physiological actions is directly associated with thyroid hormones?

  • Inhibition of protein synthesis
  • Decrease in metabolic activity in soft tissues
  • Increased uptake and utilization of glucose by tissues (correct)
  • Decreased heart rate and cardiac output

How do thyroid hormones influence cholesterol levels in the body?

  • They have no effect on cholesterol levels.
  • They decrease blood cholesterol concentration. (correct)
  • They decrease LDL receptor activity.
  • They increase cholesterol synthesis.

Which of the following is NOT a physiological effect of thyroid hormones?

<p>Decreased rate of respiration (D)</p> Signup and view all the answers

What role do thyroid hormones play in growth?

<p>They assist with myelination of nerve fibers and stimulate physical growth. (C)</p> Signup and view all the answers

Which process is NOT stimulated by thyroid hormones?

<p>Glycogen synthesis in the liver (A)</p> Signup and view all the answers

What mechanism is involved in the action of triiodothyronine (T3)?

<p>T3 forms a complex with nuclear receptors to activate DNA transcription. (A)</p> Signup and view all the answers

How do thyroid hormones affect protein metabolism?

<p>They promote both protein synthesis and catabolic reactions depending on excess. (C)</p> Signup and view all the answers

Which thyroid hormone is considered the most potent in terms of physiological action?

<p>Triiodothyronine (T3) (C)</p> Signup and view all the answers

What is the main role of calcitonin in the human body?

<p>Lower blood calcium levels (C)</p> Signup and view all the answers

How is the secretion of thyroid hormones primarily regulated?

<p>Hormonal feedback (D)</p> Signup and view all the answers

Where is thyroxine (T4) predominantly converted to triiodothyronine (T3)?

<p>Tissue cells (D)</p> Signup and view all the answers

Which of these hormones is biologically inactive?

<p>Reverse T3 (A)</p> Signup and view all the answers

What is a known effect of disturbed thyroid hormone levels in the body?

<p>Altered metabolic rate (C)</p> Signup and view all the answers

What function does parathyroid hormone serve in calcium metabolism?

<p>Increases blood calcium levels (C)</p> Signup and view all the answers

Which vitamin is essential for proper calcium metabolism?

<p>Vitamin D (C)</p> Signup and view all the answers

What is a characteristic sign of hyperthyroidism?

<p>Goiter (B)</p> Signup and view all the answers

Which of the following symptoms is associated with myxedema in adults?

<p>Cold intolerance (B)</p> Signup and view all the answers

What happens to cholesterol levels in hyperphagia with loss of body weight?

<p>Decreased cholesterol levels (A)</p> Signup and view all the answers

Which of the following is a primary cause of hypothyroidism?

<p>Chronic thyroiditis (D)</p> Signup and view all the answers

What is a consequence of protein catabolism increase in patients with hyperthyroidism?

<p>Muscle atrophy and weakness (C)</p> Signup and view all the answers

In primary hyperthyroidism, which hormone profile would be expected?

<p>High T3 and low TSH (C)</p> Signup and view all the answers

Which of these conditions is not a manifestation of hypothyroidism?

<p>Diarrhea (C)</p> Signup and view all the answers

What is a notable effect of calorogenic decrease in metabolic rate due to hypothyroidism?

<p>Weight gain despite decreased food intake (D)</p> Signup and view all the answers

What is indicated by a positive Trousseau’s sign?

<p>Carpal spasm due to blood flow restriction (B)</p> Signup and view all the answers

Which condition is associated with weak and fragile bones due to excessive bone resorption?

<p>Osteitis fibrosa cystica (C)</p> Signup and view all the answers

What is the target blood calcium level range associated with latent tetany?

<p>7 – 9 mg % (A)</p> Signup and view all the answers

What treatment is recommended during an acute attack of manifest tetany?

<p>Intravenous calcium gluconate (D)</p> Signup and view all the answers

What is the underlying cause of decreased muscle excitability in tetany?

<p>Low calcium levels (A)</p> Signup and view all the answers

Which of the following indicates increased alkaline phosphatase activity?

<p>Osteoblastic activity (B)</p> Signup and view all the answers

What common condition may result from hypercalcemia stimulating gastrin secretion?

<p>Peptic ulcer (D)</p> Signup and view all the answers

What symptom might indicate a need for calcium supplementation in a pregnant woman?

<p>Muscle weakness or cramps (A)</p> Signup and view all the answers

What is the effect of parathyroid hormone (PTH) on the kidneys?

<p>Increases calcium and magnesium reabsorption (A)</p> Signup and view all the answers

How does calcitonin affect plasma calcium levels?

<p>Decreases plasma calcium levels (C)</p> Signup and view all the answers

Which condition is indicated by low T3 and T4 levels with high TSH?

<p>Primary hypothyroidism (A)</p> Signup and view all the answers

What role does vitamin D play in calcium metabolism?

<p>Increases calcium and phosphate absorption in the intestines (A)</p> Signup and view all the answers

What is a common laboratory finding in patients with hypothyroidism?

<p>High plasma cholesterol levels (B)</p> Signup and view all the answers

What physiological change is associated with bradycardia?

<p>Decreased heart rate (C)</p> Signup and view all the answers

Which statement is true regarding the effects of parathyroid hormone on the bone?

<p>It increases the activity of osteoclasts. (C)</p> Signup and view all the answers

What is the primary action of calcitonin in bone metabolism?

<p>Inhibits osteoclast activity (D)</p> Signup and view all the answers

What effect does 1,25(OH)2D3 have on PTH secretion?

<p>It inhibits PTH secretion to lower plasma calcium levels. (B)</p> Signup and view all the answers

What is the primary cause of hypoparathyroidism?

<p>Surgical removal during thyroidectomy. (C)</p> Signup and view all the answers

Which condition is characterized by hypercalcemia and hypercalciuria?

<p>Primary hyperparathyroidism. (A)</p> Signup and view all the answers

What role does CCK have in calcium regulation?

<p>Limits the formation of PTH. (D)</p> Signup and view all the answers

What triggers tetany in individuals with hypoparathyroidism?

<p>Low ionized calcium levels. (B)</p> Signup and view all the answers

Which of the following conditions involves increased phosphate retention?

<p>Secondary hyperparathyroidism. (C)</p> Signup and view all the answers

What is a common cause of secondary hyperparathyroidism?

<p>Renal failure leading to decreased calcium reabsorption. (B)</p> Signup and view all the answers

What type of tetany occurs when calcium levels drop below 7mg %?

<p>Manifest tetany. (D)</p> Signup and view all the answers

Flashcards

What is T3?

Triiodothyronine, a thyroid hormone containing 3 iodine atoms. It's the most potent thyroid hormone, with more free form and stronger action than T4.

What is T4?

Tetraiodothyronine, a thyroid hormone containing 4 iodine atoms. It's the main form of thyroid hormone, converting to T3 in tissues.

What is Reverse T3?

A thyroid hormone that's biologically inactive. It's produced in the thyroid gland, but unlike T3 and T4, it doesn't have any physiological effects.

Where is thyroid hormone synthesized?

The synthesis of thyroid hormones is partially within cells (intracellular) and partially outside cells (extracellular).

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What is Unbound Free Thyroid Hormone?

A small amount of thyroid hormone circulates in the bloodstream unbound to proteins. This unbound form is free and can directly interact with cells.

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What is parathyroid hormone (PTH)?

A hormone secreted by the parathyroid glands. It plays a crucial role in regulating blood calcium levels by stimulating bone resorption and increasing calcium reabsorption by the kidneys.

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Where is thyroid hormone synthesized?

The synthesis of thyroid hormones is partially within cells (intracellular) and partially outside cells (extracellular).

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What is calcitonin?

A hormone secreted by the parafollicular cells (C-cells) of the thyroid gland. It lowers blood calcium levels by inhibiting bone resorption and increasing calcium excretion by the kidneys.

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Hyperphagia with weight loss

Increased appetite despite weight loss, often accompanied by increased metabolism, decreased cholesterol levels, and altered liver function.

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Cardiovascular system changes in hyperthyroidism

Increased heart rate (tachycardia), palpitations, and rapid breathing (tachypnea).

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Muscle changes in hyperthyroidism

Muscle weakness and atrophy due to increased protein breakdown (catabolism).

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Goitre

Enlargement of the thyroid gland, which can occur with normal, decreased, or increased thyroid hormone production.

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Hashimoto's disease

The most common cause of primary hypothyroidism in adults, characterized by inflammation and damage to the thyroid gland.

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Cretinism

A condition that occurs before puberty, caused by either congenital thyroid gland absence or genetic enzyme defects.

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Myxedema

A severe hypothyroidism in adults that results in slow metabolism, weight gain, and non-pitting edema (myxedema).

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Calorigenic Changes in myxedema

Characterized by decreased basal metabolic rate, cold intolerance, and weight gain despite decreased food intake.

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Parathyroid Hormone (PTH)

A hormone secreted by the parathyroid glands that regulates calcium levels in the blood. It increases calcium reabsorption in the kidneys, promotes calcium absorption in the intestines, and stimulates bone resorption.

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Calcitonin

A hormone secreted by the thyroid gland that helps lower calcium levels in the blood. It inhibits bone resorption and promotes calcium deposition in bone.

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Vitamin D

A steroid hormone that regulates calcium absorption in the intestines. It is synthesized in the skin with exposure to sunlight and also obtained through diet.

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Bone Resorption

The process by which calcium is removed from the bones.

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Bone Deposition

The process by which calcium is deposited into the bones.

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1-alpha-hydroxylase

The enzyme that converts Vitamin D to its active form, 1,25-dihydroxyvitamin D.

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Calbindin

A protein that binds calcium in the intestines, increasing calcium absorption.

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Calcium-Stimulated ATPase

An enzyme that pumps calcium into cells, increasing calcium absorption in the intestines.

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T4 Binding to Plasma Proteins

The majority (over 99%) of T4 is bound to plasma proteins, primarily thyroxine-binding globulin (TBG) synthesized by the liver. A smaller portion binds to albumin and prealbumin.

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Slow Release of T3 & T4

The release of T3 and T4 from plasma proteins to tissues is slow due to the strong affinity of the binding proteins.

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Mechanism of T3 Action

T3 binds to a nuclear receptor forming a complex that activates thyroid-regulating elements on DNA, leading to gene transcription, mRNA production, and the synthesis of specific proteins.

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Calorigenic Action of Thyroid Hormones

Thyroid hormones increase oxygen consumption and metabolic activity in most tissues.

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Thyroid Hormone Effect on Protein Metabolism

Thyroid hormones influence protein synthesis, promoting it at normal levels. However, excess thyroid hormones can lead to protein breakdown (catabolism).

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Thyroid Hormone Effect on Fat Metabolism

Thyroid hormones increase lipolysis (breaking down fat) and mobilization of fat from storage tissues.

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Thyroid Hormone Effect on Carbohydrate Metabolism

Thyroid hormones increase glucose uptake and utilization by tissues, stimulate liver glycogenolysis and gluconeogenesis, and enhance intestinal glucose absorption.

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Thyroid Hormones and Vitamin Metabolism

Thyroid hormones are necessary for the conversion of carotene to Vitamin A in the liver. They also increase the body's need for vitamins due to the general increase in enzyme activity.

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Hypophosphatemia

Low phosphorus levels in the blood. It can lead to weakened bones, kidney stones, muscle weakness, constipation, and nervous system problems.

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Hyperphosphaturia

Increased excretion of phosphorus in the urine. It can be a symptom of hypophosphatemia.

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Carpopedal Spasm

A specific hand posture seen in hypocalcemia. It involves flexion of the wrist and metacarpophalangeal joints, extension of interphalangeal joints, and adduction of the thumb.

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Osteitis Fibrosa Cystica

A form of bone disease caused by low phosphorus levels. It's characterized by increased bone resorption and osteoclastic activity, leading to weakened and fragile bones.

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Trousseau's Sign

Positive Trousseau's sign is a sign of hypocalcemia, where the hand flexes involuntarily when blood flow to the forearm is restricted.

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Chvostek's Sign

Positive Chvostek's sign is another sign of hypocalcemia. It involves the facial muscle twitching when the facial nerve is tapped.

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Latent Tetany

A condition where the body does not show any signs of tetany at rest, but symptoms appear under stress or increased calcium needs.

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Manifest Tetany

A condition where the body shows the classic symptoms of tetany like muscle spasms, cramps, and seizures.

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Tetany

A condition characterized by increased neuromuscular excitability due to low ionized calcium in the blood. This is often caused by decreased calcium levels, vitamin D deficiency, or kidney failure.

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Primary Hyperparathyroidism - Adenoma

A type of hyperparathyroidism where there is an overproduction of parathyroid hormone by a single adenoma (benign tumor) within the parathyroid gland.

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Primary Hyperparathyroidism - Hyperplasia

A type of hyperparathyroidism where the parathyroid glands are enlarged due to excessive stimulation, often in response to chronic low calcium levels in the blood.

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Secondary Hyperparathyroidism

A type of hyperparathyroidism that occurs as a consequence of chronic conditions like vitamin D deficiency, kidney failure, or malnutrition, where the body tries to compensate by increasing parathyroid hormone production.

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Tertiary Hyperparathyroidism

A variant of secondary hyperparathyroidism where even after the underlying low calcium condition is corrected, the parathyroid glands remain overactive and continue to produce excessive parathyroid hormone.

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1,25(OH)2D3 (Calcitriol)

The active form of vitamin D, produced in the kidneys. It plays a crucial role in regulating calcium levels by increasing calcium absorption from the gut.

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Hypoparathyroidism

A condition where the parathyroid glands are underactive, leading to low calcium levels in the blood. It can be caused by surgery, autoimmune disorders, or genetic conditions.

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Study Notes

Thyroid Hormone Actions

  • Thyroid hormones (primarily T3 and T4) affect numerous bodily functions
  • They increase metabolic rate, impacting cellular processes like oxygen consumption
  • Increased metabolic activity affects appetite, digestion, and respiration
  • Increased heart rate and blood pressure are observed as well
  • Essential for growth, particularly in bones and soft tissues
  • Thyroid hormones influence protein synthesis
  • Excessive thyroid hormones can lead to a catabolic effect

Thyroid Hormone Synthesis and Regulation

  • Thyroid hormones are synthesized from tyrosine and iodine
  • Thyroid hormone synthesis involves both intracellular and extracellular steps
  • Thyroid hormone secretion is primarily regulated through a hypothalamic-pituitary axis
  • Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates thyroid-stimulating hormone (TSH) release from the pituitary
  • TSH further stimulates thyroid hormone synthesis and secretion
  • Plasma binding proteins (primarily TBG) transport most thyroid hormones in the blood
  • Thyroid hormone regulation involves feedback loops responding to hormone levels in the blood

Calcium Metabolism

  • Parathyroid hormone (PTH) is crucial for calcium homeostasis
  • It increases blood calcium levels by targeting the kidneys, intestines, and bones
  • PTH increases calcium reabsorption in the kidneys and calcium absorption in the intestines
  • It enhances bone resorption promoting calcium release, mainly through osteoclast activation
  • Calcitonin is a hormone that opposes PTH's actions on calcium homeostasis
  • It reduces blood calcium levels by inhibiting bone resorption

Vitamin D and Calcium

  • Vitamin D plays a significant role in calcium absorption in the intestines and calcium balance
  • Vitamin D is converted to its active form (1,25-dihydroxyvitamin D3) in the kidneys
  • Vitamin D promotes calcium uptake primarily in the intestines
  • Vitamin D is partly stimulated by parathyroid hormone to contribute in the regulation of calcium balance

Disorders of Thyroid Function and Calcium Metabolism

  • Hypothyroidism is characterized by low thyroid hormone levels and results in a decreased metabolic rate
  • Hyperthyroidism is characterized by high thyroid hormone levels and results in an increased metabolic rate
  • Hypoparathyroidism is characterized by low PTH levels leading to decreased calcium and increased phosphorus
  • Hyperparathyroidism is characterized by high PTH levels leading to increased calcium and decreased phosphorus

Thyroid Hormone Secretion Regulation

  • A rise in plasma calcium inhibits parathyroid hormone (PTH) secretion
  • Certain hormones like estrogen and prolactin stimulate calcitonin release
  • A rise in plasma phosphate stimulates calcitonin release
  • Gastrointestinal hormones like gastrin and cholecystokinin regulate calcitonin release

Thyroid Gland Disorders

  • Graves disease: autoimmune disorder causing hyperthyroidism
  • Hashimoto's thyroiditis: autoimmune disorder causing hypothyroidism
  • Goiter: enlargement of the thyroid gland, often associated with iodine deficiency or certain disorders

Calcium Metabolism Disorders

  • Hypoparathyroidism: low parathyroid hormone levels, often resulting in low calcium
  • Hyperparathyroidism: high parathyroid hormone levels, often resulting in high calcium

Physiological Actions of Thyroid Hormones

  • Increased oxygen consumption, leading to higher metabolic rate and heat production (calorigenic effect)
  • Increase in protein synthesis
  • Increase in carbohydrate metabolism
  • Stimulation of growth and development, particularly in bones and soft tissues
  • Effects on the cardiovascular system (heart rate, cardiac output, and blood pressure)
  • Regulation of CNS (central nervous system) function and development

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Thyroid Hormones-Calcium Metabolism (Medical Physiology-ERD-313 PDF)

Description

This quiz covers the actions, synthesis, and regulation of thyroid hormones, specifically T3 and T4. It explores their effects on metabolism, growth, and various bodily functions. Understand how thyroid hormone synthesis is regulated by the hypothalamic-pituitary axis and its implications for health.

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