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Questions and Answers
Which symptom is NOT commonly associated with hyperthyroidism?
Which symptom is NOT commonly associated with hyperthyroidism?
What is a significant complication of hypothyroidism?
What is a significant complication of hypothyroidism?
What is the primary metabolic function of T3 and T4 hormones?
What is the primary metabolic function of T3 and T4 hormones?
Which action does parathyroid hormone (PTH) have regarding calcium levels?
Which action does parathyroid hormone (PTH) have regarding calcium levels?
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Cretinism can result from severe deficiency of which hormone?
Cretinism can result from severe deficiency of which hormone?
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What is a direct effect of calcitonin on calcium levels in the blood?
What is a direct effect of calcitonin on calcium levels in the blood?
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Which of the following statements about hyperthyroidism is true?
Which of the following statements about hyperthyroidism is true?
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The primary function of parathyroid hormone in the kidneys includes
The primary function of parathyroid hormone in the kidneys includes
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What is one of the primary metabolic effects of T3 and T4 on carbohydrate metabolism?
What is one of the primary metabolic effects of T3 and T4 on carbohydrate metabolism?
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Which of the following symptoms is commonly associated with hyperthyroidism?
Which of the following symptoms is commonly associated with hyperthyroidism?
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What complication is often linked to hypothyroidism?
What complication is often linked to hypothyroidism?
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What is the result of the thyrotoxic myopathy in relation to protein metabolism?
What is the result of the thyrotoxic myopathy in relation to protein metabolism?
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Thyroid hormones primarily function to regulate which of the following?
Thyroid hormones primarily function to regulate which of the following?
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How do thyroid hormones affect cardiovascular system functioning?
How do thyroid hormones affect cardiovascular system functioning?
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Which of the following statements is true regarding the transport of thyroid hormones?
Which of the following statements is true regarding the transport of thyroid hormones?
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What role does calcitonin play in the body?
What role does calcitonin play in the body?
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Study Notes
Thyroid Gland Anatomy
- The thyroid gland is composed of follicles surrounded by a cellular layer.
- These follicles are filled with a protein substance called colloid.
- The gland is located in the neck region, anterior to the trachea.
- It's composed of the right and left lobes, connected by the isthmus.
- Additional structures like the pyramidal lobe may be present.
- Anatomically related structures include the hyoid bone, thyroid cartilage, cricoid cartilage and cricothyroid ligament.
Thyroid Hormones
- Tetraiodothyronine (T4), also known as thyroxine, and triiodothyronine (T3) are produced by follicular cells.
- Calcitonin is produced by parafollicular cells.
- T4 is converted to T3 in the target tissues.
- The production of these hormones involves chemical reactions, creating the functional variants of T3 and T4, based on tyrosine molecules and Iodine uptake.
Thyroid Hormone Synthesis
- Steps in thyroid hormone synthesis include iodide uptake, oxidation, organification, coupling, secretion, fusion, and release.
- Key intermediates and enzymes are involved in the steps of hormone synthesis including peroxidases, iodination reactions and coupling of MIT and DIT.
Thyroid Hormone Coupling
- MIT + DIT → T3
- DIT + DIT → T4
- MIT + DIT → rT3
Thyroid Hormone Transport
- Thyroid hormones are transported in the blood primarily bound to carrier proteins.
- Thyroxine-binding globulin (TBG) carries 60% of the hormone.
- Thyroxine-binding prealbumin (TBPA) carries 30% of the hormone.
- Albumin carries 10% of the hormone.
Thyroid Hormone Levels
- Bound T4 (total T4): 8 µg/dL in plasma.
- Unbound T4 (free T4): 2 ng/dL in plasma.
Specific Actions of Thyroid Hormones
- Regulates basal metabolic rate (BMR)
- Increases oxygen consumption in most target tissues
- Permissive actions - increase sensitivity of target tissues to catecholamines for lipolysis, glycogenolysis, and gluconeogenesis.
Growth Promotion
- Thyroid hormones promote eruption of teeth, growth and development of the central nervous system (CNS), formation of ossification centers, and general body growth (along with growth hormone).
Carbohydrate Metabolism
- Increases carbohydrate absorption in the gastrointestinal tract (GIT), promotes glycogenolysis and gluconeogenesis.
- Elevated blood glucose levels, known as "thyroid diabetes," can be observed.
Fat Metabolism
- Decreases circulating cholesterol, plasma fatty acids, and phospholipid levels
Protein Metabolism
- Increases breakdown of muscle protein, increases creatine excretion, and decreases phospholipid levels
Effects on CVS
- Increases force of contraction, heart rate, and systolic blood pressure.
Effects on CNS
- Promotes development and growth of the CNS, myelination, and synaptogenesis
- Improves mental functions and intelligence
- Also affects behavior
Thyroid Hormone Regulation
- Controlled by the hypothalamus-pituitary axis pathway.
- TRH released by hypothalamus stimulates TSH release by the pituitary, which in turn stimulates the thyroid to release T4 and T3.
- Negative feedback mechanisms regulate the hormone production.
Hyperthyroidism
- Other names include toxic goiter, thyrotoxicosis, and Graves' disease.
- Symptoms include high excitability, intolerance to heat, increased sweating, weight loss/muscle weakness, diarrhea, nervousness, extreme fatigue, and hand tremors.
Myxedema
- Almost total thyroid deficiency in adults.
- Characterized by facial swelling, increased blood cholesterol levels potentially leading to atherosclerosis and coronary artery disease.
Cretinism
- Due to extreme hypothyroidism during prenatal or neonatal/ early childhood.
- Sluggish movement, retarded physical and mental growth.
- Neurological impairment (specifically developmental delay), short appearance and obesity
Parathyroid Hormone (PTH)
- Released by chief cells in the parathyroid glands.
- Functions: increases blood calcium levels by decreasing calcium excretion in the urine, increasing phosphate excretion in the urine, increasing bone resorption, and activating vitamin D3.
- Actions: activation of osteocytes to promote calcium and phosphate absorption, promotion of osteoclast proliferation and absorption.
- Effects on Kidneys: Decreases calcium excretion & phosphate excretion
- Effects on Intestines: Increases calcium and phosphate absorption
Vitamin D
- Vitamin D synthesis involves UV light, cholecalciferol (vitamin D3) conversion, and kidney activation, producing 1,25-dihydroxycholecalciferol (calitriol).
- Calcitriol is the active form of vitamin D
- Vitamin D plays a role in calcium homeostasis.
Calcitonin
- A peptide hormone secreted by parafollicular cells of the thyroid gland, acting opposite to PTH.
- Decreases plasma calcium concentration
- Decreases activity of osteoclasts and osteolytic effect of osteocytic membrane.
- Decreases newer formation of osteoclast cells.
Hypoparathyroidism
- Low parathyroid hormone levels.
- Ca levels drop to 6-7 mg/dL, causing laryngeal muscle spasms, obstructing respiration and potentially causing death, also difficulty absorbing nutrients.
- Treatment involves vitamin D and calcium supplementation.
Hyperparathyroidism
- Tumors in the parathyroid glands, usually more prevalent in females.
- Elevated plasma calcium levels (12-15 mg/dL) can cause CNS and PNS depression, muscle weakness, constipation, abdominal pain, peptic ulcers, and reduced heart relaxation.
- Can lead to osteitis fibrosa cystica (cystic bone disease)
- Rickets in children and osteomalacia in adults
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Description
Explore the structure and function of the thyroid gland, including its anatomy, the hormones it produces, and the synthesis process. This quiz covers essential aspects of T4 and T3, as well as related anatomical features and hormone reactions.
