Thyroid Gland Anatomy and Function

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Questions and Answers

What is the role of Thyroid-Stimulating Hormone (TSH)?

Inhibits the release of thyroid hormones

Regulates thyroid hormone production, secretion, and growth (correct)

Stimulates the adrenal gland function

Increases metabolic rate directly

What can cause thyrotoxicosis?

Insufficient thyroid hormone production

Decreased pituitary activity

Increased adrenaline levels

High levels of circulating thyroid hormones (correct)

Which symptom is NOT associated with hyperthyroidism?

Rapid heart rate

Bulging eyes

Weight gain (correct)

Sweating

Which sign indicates a lag in the upper eyelid when looking downwards?

Von Graefe's sign Signup and view all the answers

Which of the following changes can occur in individuals with hypothyroidism?

Mental fog Signup and view all the answers

How are abnormalities in thyroid function typically evaluated?

By measuring total T4 and total T3 levels Signup and view all the answers

What is a typical characteristic of hyperthyroidism?

Weight loss Signup and view all the answers

What is a common psychological symptom associated with hyperthyroidism?

Nervousness Signup and view all the answers

Which hormone is primarily responsible for increasing blood glucose levels?

Growth hormone Signup and view all the answers

What is a major clinical feature of acromegaly?

Coarse facial features Signup and view all the answers

What diagnostic tool is most useful for assessing hyperthyroidism through iodine uptake?

Radio-active iodine uptake Signup and view all the answers

Which of the following is NOT a symptom of Cushing syndrome?

Weight loss Signup and view all the answers

What can cause excessive growth hormone secretion leading to acromegaly?

Pituitary adenoma Signup and view all the answers

Which gland is responsible for secreting cortisol?

Adrenal cortex Signup and view all the answers

What complication is commonly associated with acromegaly due to growth hormone excess?

Joint pain Signup and view all the answers

Which of the following is a common cause of Cushing syndrome?

Iatrogenic steroid therapy Signup and view all the answers

What is the primary secretory product of the thyroid gland?

Thyroxine (T4) Signup and view all the answers

Which of the following hormones is primarily responsible for regulating skeletal maturation?

Triiodothyronine (T3) Signup and view all the answers

How is the majority of circulating T3 produced?

Conversion from T4 in peripheral tissues Signup and view all the answers

What percentage of circulating T4 is typically bound to plasma carrier proteins?

More than 99% Signup and view all the answers

Which of the following is NOT a function of thyroid hormones?

Decreasing basal metabolic rate Signup and view all the answers

What is the recommended minimum daily intake of iodine?

150 µg/day Signup and view all the answers

Which plasma carrier protein accounts for the largest percentage of T4 binding?

Thyroxine-binding globulin (TBG) Signup and view all the answers

What role does free hormone play in thyroid function?

It has metabolic activity and physiologic effects Signup and view all the answers

What impact do thyroid hormones have on serum cholesterol levels?

Decrease serum cholesterol Signup and view all the answers

Which of the following describes the overall function of the thyroid gland?

To produce hormones aiding in metabolism and development Signup and view all the answers

Which symptom is NOT commonly associated with adrenal malignancy?

Salt craving Signup and view all the answers

What is a common sign of adrenal hypofunction?

Hyperpigmentation Signup and view all the answers

Which of the following is a characteristic feature of Cushing's syndrome caused by ectopic ACTH secretion?

Hirsutism Signup and view all the answers

What is the main stimulus for the secretion of parathormone (PTH)?

Hypocalcemia Signup and view all the answers

Which symptom is likely to be seen in primary hyperparathyroidism due to hypercalcemia?

Nausea Signup and view all the answers

What effect does adrenal malignancy have on muscle strength?

Weakens and thins muscles Signup and view all the answers

Which of the following conditions could lead to primary adrenal insufficiency?

Granulomatous disease Signup and view all the answers

What is a potential consequence of long-term high levels of PTH?

Decreased bone density Signup and view all the answers

What clinical manifestation is NOT associated with hypercalcemia?

Bronchospasm Signup and view all the answers

Which condition is characterized by failure in the secretion of PTH?

Hypoparathyroidism Signup and view all the answers

What abnormal laboratory finding is indicative of hypoparathyroidism?

Increased phosphate Signup and view all the answers

Which of the following is NOT a symptom of hypercalcemia?

Stridor Signup and view all the answers

Which surgical procedure could lead to hypoparathyroidism?

Thyroidectomy Signup and view all the answers

What condition may result from calcium deposition in renal tubules?

Nephrocalcinosis Signup and view all the answers

Which of the following is a symptom of hypoparathyroidism?

Numbness and tingling Signup and view all the answers

What is the effect of prolonged severe hypomagnesemia on PTH secretion?

Decreased PTH secretion Signup and view all the answers

Study Notes

Anatomy of the Thyroid Gland

The thyroid gland's key function is to secrete thyroid hormones and calcitonin.
The thyroid gland is composed of follicles, which serve as the functional units.

The Thyroid Produces and Secretes 2 Metabolic Hormones

These hormones are thyroxine (T4) and triiodothyronine (T3).
They are essential for cell homeostasis, influencing cell differentiation, growth, and metabolism.

Iodine Sources

Iodine is a trace mineral obtained through foods like seafood, bread, and dairy products, iodized salt, or supplements.
The recommended daily intake is at least 150 micrograms.

Production of T4 and T3

T4 is primarily secreted by the thyroid gland, while T3 is produced through two processes:
- 80% of circulating T3 originates from the deiodination of T4 in peripheral tissues.
- 20% comes from direct thyroid secretion.

Carriers for Circulating Thyroid Hormones

Over 99% of circulating T4 and T3 are bound to plasma carrier proteins:
- Thyroxine-binding globulin (TBG) accounts for 75%.
- Transthyretin (TTR), also known as thyroxine-binding prealbumin (TBPA) contributes 10-15%.
- Albumin binds about 7%.
Carrier proteins can be influenced by physiological changes, medications, and diseases.
Only unbound (free) hormone exhibits metabolic activity and physiological effects.
- Free hormone constitutes a small percentage of total hormone in plasma (around 0.03% T4; 0.3% T3).

Effects of Thyroid Hormone

Thyroid hormone plays a crucial role in fetal brain and skeletal maturation.
It increases the basal metabolic rate.
It exerts inotropic and chronotropic effects on the heart.
It enhances sensitivity to catecholamines.
It stimulates gut motility.
It increases bone turnover.
It raises serum glucose levels and decreases serum cholesterol.

Thyroid Hormone is Critical for Normal Bone Growth and Development

T3 is essential for skeletal maturation at the growth plate.
- It regulates the expression of factors contributing to linear growth.
- It participates in osteoblast differentiation and proliferation, as well as chondrocyte maturation leading to bone ossification.

Control of Thyroid Function: Hypothalamic-Pituitary-Thyroid Axis

The hypothalamus secretes Thyroid Releasing Hormone (TRH).
TRH stimulates the anterior pituitary to release Thyroid Stimulating Hormone (TSH).
TSH, in turn, regulates thyroid hormone production, secretion, and growth.
T4 and T3 exert negative feedback on TSH.

Thyroid-Stimulating Hormone (TSH)

Regulates thyroid hormone production, secretion, and growth.
Subject to negative feedback regulation by T4 and T3.

Thyrotoxicosis & Hyperthyroidism

Thyrotoxicosis is a clinical syndrome characterized by high levels of circulating thyroid hormones.
Hyperthyroidism is a condition where the thyroid gland is overactive.

Hyperthyroid Symptoms

Weight loss despite increased appetite.
Heat intolerance.
Increased sweating.
Palpitations and rapid heart rate.
Tremor.
Nervousness and anxiety.
Insomnia.
Diarrhea.
Muscle weakness.
Goiter (enlargement of the thyroid gland).
Eye problems (exophthalmos, lid lag).
Hair loss.
Menstrual irregularities.
Fatigue.

Hypothyroid Symptoms

Weight gain despite decreased appetite.
Cold intolerance.
Constipation.
Fatigue and lethargy.
Muscle aches and weakness.
Dry skin and hair.
Bradycardia (slow heart rate).
Depression.
Goiter.
Memory impairment.
Hoarseness.
Menstrual irregularities.

Evaluation of Thyroid Function

Total T4 (normal: 4-12 ug/dl).
Total T3 (normal: 80-120 ng/dl).
Free T3 and T4 levels.
Radioactive iodine uptake (RAI): Normal uptake (24 hours after administration) is 5-30% of the administered dose.
Serum TSH levels.
Thyroid ultrasound.
Fine-needle aspiration biopsy.

Growth Hormone Physiology

Increases blood glucose.
Enhances protein synthesis.
Increases free fatty acids (FFA).

Acromegaly

Characterized by excessive growth caused by overproduction of GH.
Usually due to a benign pituitary tumor (adenoma).

Acromegaly Clinical Picture

Coarse facial features, including exaggerated supraorbital ridges.
Enlarged soft tissue in the nose and lips.
Enlarged and furrowed tongue.
Overgrowth of the jaw bone (prognathism).
Enlarged hands and feet.
Coarse body hair growth with skin thickening and darkening.
Increased sweating.
Deep and husky voice due to thickened cartilage in the larynx.
Joint pain.
Cardiomegaly, hypertension, and hepatomegaly.
Nerve compression leading to sensory disturbances and weakness in the arms and legs.
Severe headaches.
Irregular menstrual cycles.

Adrenal Gland Disorders

The adrenal gland's cortex secretes cortisol, mineralocorticoids, and androgens, while the medulla secretes adrenaline (epinephrine).

Functions of Adrenocortical Hormones

Involved in complex nutrient processing and utilization, including carbohydrates, fats, and proteins.
Essential for the normal functioning of the circulatory system and heart.
Regulate muscle function.
Contribute to blood cell production.
Maintain the skeletal system.
Support proper brain and nerve function.
Regulate the immune system's responses.

Cushing's Syndrome

Excess cortisol circulating in the bloodstream.

Cushing's Syndrome Etiologies

Iatrogenic: Steroid therapy is the most common cause.
Pituitary adenoma (ACTH) leading to adrenal cortex hyperplasia.
Adrenal cause:
- Adrenal adenoma.
- Adrenal hyperplasia.
- Adrenal malignancy.
Ectopic source (ACTH): Small cell carcinoma of the lung.

Cushing's Syndrome Symptoms

Mood changes, including depression and euphoria.
Easy bruising.
Weakness.
Weight gain.
Amenorrhea.
Back pain.

Cushing's Syndrome Signs

Weight gain, particularly in the face ("moon face"), trunk ("truncal obesity"), upper back, and back of the neck ("buffalo hump").
Striae rubra (purple and pink stretch marks).
High blood pressure.
Weak and thinning bones (osteoporosis).
Weak muscles.
Thin and fragile skin with bruising and slow healing tendencies.
Potential development of diabetes.
Increased susceptibility to infections.
Hirsutism in women: abnormal hair growth in areas like the beard and mustache, accompanied by hair loss on the head (receding hairline).

Adrenal Gland Hypofunction

Etiology:
- Secondary: Pituitary insufficiency.
- Primary: Addison's disease (primary adrenal failure).
  - Hypoplasia or destruction of the adrenal glands due to:
    - Granulomatous disease (e.g., tuberculosis, fungus).
    - Amyloidosis.
    - Hemochromatosis.
    - Tumors.
    - Autoimmune processes.

Adrenal Gland Hypofunction Symptoms

Weakness, tiredness, and dizziness.
Fatigue or lethargy.
Nausea, vomiting, and weight loss.
Diarrhea and dehydration.
Salt craving.
Abdominal pain.

Adrenal Gland Hypofunction Signs

Hyperpigmentation:
- Skin darkening, resembling tanning, on both sun-exposed and unexposed areas.
- Pigmented buccal mucosa.
Hypotension.
Reduced hair growth.
Signs of dehydration.

Parathyroid Glands

Four small glands located in the neck at the posterior surface of the thyroid gland.
Secrete parathyroid hormone (PTH), which maintains calcium homeostasis in the blood.

Stimuli for PTH Secretion

Hypocalcemia (low calcium levels in the blood) is the primary stimulus.

Hyperparathyroidism

Majority of patients are asymptomatic (50%).
Non-specific symptoms may include weakness and fatigue.
Symptoms related to hypercalcemia:
- Anorexia, nausea, and vomiting.
- Dyspepsia.
- Constipation.
- Increased incidence of peptic ulcers and acute pancreatitis.
- Interstitial nephritis: Calcium deposition in the renal tubules leading to inability to concentrate urine, resulting in polyuria, polydipsia, and nocturia.
- Increased susceptibility to renal stones (calcium oxalate and calcium phosphate stones), causing renal colic and repeated UTIs.
- Nephrocalcinosis due to whole kidney calcification.
- Impaired mentation, memory loss, emotional lability, depression, anosmia, drowsiness, stupor, and coma.
- Weakness (proximal muscles), arthralgia.
- Severe pruritus (skin calcification).
- Joint pains.
- Rare cases of pathological fractures.
- Calcification at the corneo-scleral junction, leading to keratitis and corneal opacity.
- Solitary bone resorption causing solitary cysts, brown tumors of the mandible, or subperiosteal erosion of the phalanges (osteitis fibrosa cystica).

Hyperparathyroidism Diagnosis

Hypercalcemia (Normal Ca = 9-11 mg/100cc).
Hypophosphatemia (Normal = 2.5-4 mg/100cc).
Increased alkaline phosphatase.
Increased PTH levels.
Urinalysis and kidney X-rays.
Bone density tests.

Hyperparathyroidism Treatment

Medical: Management of hypercalcemia.
Surgical removal of the affected parathyroid gland.

Hypoparathyroidism

Failure of PTH secretion due to:
- Surgical: Following thyroidectomy, neck exploration, or removal of neck malignancies.
- Transient: After removal of a parathyroid adenoma.
- Idiopathic (autoimmune).
- Prolonged severe hypomagnesemia.
Clinical picture:
- Asymptomatic or manifestations of hypocalcemia.
- Neuromuscular activity disturbances:
  - Numbness and tingling around the mouth and lips.
  - Paresthesias in the hands and feet.
  - Bronchospasm or laryngeal spasm (stridor).
- Carpopedal spasm (spasmodic contractions of the hands and feet).

Hypoparathyroidism Diagnosis

Decreased serum calcium levels.
Increased phosphate levels in the absence of renal failure.
Low PTH levels: Diagnostic of hypoparathyroidism.

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Description

This quiz covers the anatomy and physiology of the thyroid gland, detailing its essential functions, the production and secretion of T4 and T3 hormones, and the importance of iodine as a dietary mineral. Test your understanding of how these elements contribute to metabolism and overall health.