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Questions and Answers
What is the primary cause of hyperthyroidism in most cases?
What is the primary cause of hyperthyroidism in most cases?
Which of the following symptoms is associated with Grave's disease?
Which of the following symptoms is associated with Grave's disease?
What effect does radioactive iodine treatment have on the thyroid gland?
What effect does radioactive iodine treatment have on the thyroid gland?
In primary hyperthyroidism, what is the typical level of TSH?
In primary hyperthyroidism, what is the typical level of TSH?
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What treatment option might be considered for pregnant women with hyperthyroidism?
What treatment option might be considered for pregnant women with hyperthyroidism?
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What is most likely to be required after surgery to remove the thyroid gland?
What is most likely to be required after surgery to remove the thyroid gland?
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Which of the following accurately describes secondary hyperthyroidism?
Which of the following accurately describes secondary hyperthyroidism?
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What is a potential consequence of untreated hyperthyroidism?
What is a potential consequence of untreated hyperthyroidism?
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What is the primary characteristic of toxic adenomas?
What is the primary characteristic of toxic adenomas?
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What is a common cause of temporary hyperthyroidism?
What is a common cause of temporary hyperthyroidism?
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Which symptom is NOT commonly associated with hyperthyroidism?
Which symptom is NOT commonly associated with hyperthyroidism?
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Which of the following treatments is NOT typically used to manage hyperthyroidism?
Which of the following treatments is NOT typically used to manage hyperthyroidism?
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What is the role of beta blocker medicines in the treatment of hyperthyroidism?
What is the role of beta blocker medicines in the treatment of hyperthyroidism?
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What potential complication can hyperthyroidism cause in pregnant women?
What potential complication can hyperthyroidism cause in pregnant women?
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Which diagnostic test is NOT traditionally associated with hyperthyroidism?
Which diagnostic test is NOT traditionally associated with hyperthyroidism?
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Graves' ophthalmopathy is primarily characterized by which of the following symptoms?
Graves' ophthalmopathy is primarily characterized by which of the following symptoms?
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What is the primary cause of hypothyroidism in areas with sufficient dietary iodine?
What is the primary cause of hypothyroidism in areas with sufficient dietary iodine?
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Which of the following symptoms is NOT typically associated with hypothyroidism?
Which of the following symptoms is NOT typically associated with hypothyroidism?
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Which of these conditions can lead to hyperthyroidism due to an issue in the thyroid gland?
Which of these conditions can lead to hyperthyroidism due to an issue in the thyroid gland?
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Which of the following is a direct symptom of hyperthyroidism?
Which of the following is a direct symptom of hyperthyroidism?
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What is the thyroid hormone level range considered normal for adults?
What is the thyroid hormone level range considered normal for adults?
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Which of the following could be a cause of secondary hyperthyroidism?
Which of the following could be a cause of secondary hyperthyroidism?
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What is a common sign of severe hypothyroidism known as myxedema?
What is a common sign of severe hypothyroidism known as myxedema?
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Which of these is a female-specific symptom of hypothyroidism?
Which of these is a female-specific symptom of hypothyroidism?
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Study Notes
Hypothalamus-Pituitary-Thyroid Axis
- The hypothalamus, pituitary, and thyroid gland work together in a feedback loop to regulate thyroid hormone production.
- TRH (Thyrotropin-releasing hormone) is released by the hypothalamus.
- TRH travels to the anterior pituitary.
- TSH (Thyroid-stimulating hormone) is released by the anterior pituitary.
- TSH travels to the thyroid gland.
- The thyroid gland releases T4 (thyroxine) and T3 (triiodothyronine).
- Thyroid hormones T3 & T4 stimulate metabolism.
- Thyroid hormones exert negative feedback on the hypothalamus and anterior pituitary.
Thyrotropin-Releasing Hormone (TRH)
- TRH is synthesized in the paraventricular nuclei of the hypothalamus.
- It travels to the anterior pituitary via the hypophyseal portal system.
- TRH stimulates the release of TSH from the anterior pituitary.
- TRH has a half-life of approximately 6 minutes in the blood.
- TRH is a tripeptide.
- TRH has anti-depressant and anti-suicidal properties.
- TRH receptor (TRHR) is a G protein-coupled receptor.
- TRH activates phospholipase C, leading to the formation of IP3 and DAG.
- This increases cytoplasmic calcium ion concentrations, stimulating the exocytosis of TSH into the blood.
Thyroid Stimulating Hormone (TSH)
- TSH is a glycoprotein hormone.
- TSH is produced by thyrotroph cells in the anterior pituitary.
- TSH stimulates the thyroid gland to produce T4 and T3.
- TSH levels are higher in children than adults.
- Normal range in adults is 0.4–2.5 µIU/mL.
- TSH stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism.
- TSH is secreted throughout life but particularly reaches high levels during periods of rapid growth and development, as well as in response to stress.
- TSH regulates thyroid hormone synthesis.
- Increased TSH can lead to increased thyroid follicular cells (size and number), potentially resulting in goiter.
- TSH receptors are found mainly on thyroid follicular cells and are G protein-coupled receptors.
Thyroid Gland
- The thyroid gland is one of the largest endocrine glands in the body.
- It weighs approximately 20 grams.
- Thyroid gland size depends on age, sex, and physiological condition (e.g., pregnancy, lactation).
- The thyroid gland is located in the neck, just below the larynx, on either side of and anterior to the trachea.
- The thyroid gland has a rich blood supply.
- Blood supply is provided by various arteries and veins.
- The thyroid gland is made up of follicles, which are the functional units.
- Each follicle is spherical and filled with colloid.
- Follicular cells line the follicles.
- Follicular cells secrete into the interior of the follicles.
- Parafollicular cells (C cells) are located between follicles.
- Parafollicular cells (C cells) produce calcitonin.
Thyroid Hormones Secretion
- T3 and T4 are important thyroid hormones.
- T4 (thyroxine) accounts for 93% of the secretion.
- T3 (triiodothyronine) accounts for 7% of the secretion.
- Thyroid hormones are stored in follicles for 2 to 3 months.
- T3 is the active form of T4.
- T3 is more potent than T4 but present in smaller quantities in the blood.
- Almost all T4 is converted to T3 in tissues.
- Most T3 is produced by peripheral tissues (primarily the liver and kidneys) through the deiodination of T4.
Control of Thyroid Hormone Secretion
- Thyroid hormone (TH) secretion is tightly regulated by negative feedback loops involving TRH and TSH.
- THs negatively feedback on both the hypothalamus and anterior pituitary to inhibit further TRH and TSH release.
- The hypothalamus produces TRH, which stimulates the anterior pituitary to produce TSH.
- TSH stimulates the thyroid gland to produce T3 and T4.
- T3 and T4 then inhibit further production of TRH and TSH.
Mechanism of Action of Thyroid Hormones
- Thyroid hormones are lipophilic (fat-soluble).
- THs receptors are located within the target cells nuclei.
- T3 binds to nuclear receptors, triggering gene expression.
Thyroid Hormone Transport
- Most thyroid hormones (T3 and T4) are bound to transport proteins (e.g., thyroxine-binding globulin, albumin) in the blood.
- Only a small amount of T3 and T4 circulate freely in the blood.
Iodine Metabolism
- Iodine is an essential component of thyroid hormones.
- Dietary iodine is absorbed in the GI tract.
- The transport of iodide into the follicular cells is dependent on a Na+/I- cotransport system.
- Iodine is oxidized to iodine (I2).
- Iodine then attaches to tyrosine within thyroglobulin.
- Monoiodotyrosine (MIT) and diiodotyrosine (DIT) are formed.
- MIT and DIT combine to form T3 and T4.
Goiter/Swollen Neck
- Goiter means there is a swollen neck.
- Goiter commonly happens when there is a lack of iodine which the thyroid can no longer produce sufficient T3 & T4.
- This lack of production causes excessive stimulation from TSH that leads to the growth of the thyroid gland.
Hypothyroidism
- Hypothyroidism is a condition where the thyroid gland does not produce enough thyroid hormones.
- Causes include iodine deficiency, autoimmune diseases (like Hashimoto's thyroiditis), and certain medications.
Hyperthyroidism
- Hyperthyroidism is the opposite of hypothyroidism.
- Hyperthyroidism is an overproduction of Thyroid hormones T3 and T4.
- Hyperthyroidism can be caused by Graves' disease (autoimmune disease), toxic nodules, and certain types of thyroiditis.
Myxedema
- Myxedema is a life-threatening complication of untreated hypothyroidism.
- Symptoms include severe swelling of the face, slow breathing, a low body temperature etc
Hypothyroidism in Children (Cretinism)
- In children, hypothyroidism during postnatal development (Cretinism) can result in severe mental retardation and short stature.
Normal Thyroid Hormone Levels
- Normal ranges vary by age.
- Adults have a normal range of 0.4-4.0 mIU/L.
Diagnosis Hypo-/Hyperthyroidism
- Medical history and physical exam
- Thyroid function tests (TSH, T3, T4)
- Imaging tests (thyroid scan, ultrasound)
- Radioactive iodine uptake test
Treatments for Hypo-/Hyperthyroidism
- Hypothyroidism: hormone replacement therapy (e.g., levothyroxine)
- Hyperthyroidism: antithyroid medications, radioactive iodine, or surgery.
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Description
Test your knowledge on the hypothalamus-pituitary-thyroid axis and the role of TRH in regulating thyroid hormone production. This quiz covers hormone pathways, functions, and the feedback mechanisms involved in thyroid regulation. Ideal for students studying human physiology or endocrinology.
