Physiology and Histology of Thyroid Gland

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Questions and Answers

Which component is part of the structure of thyroid-stimulating hormone?

One α-chain and one β-chain (correct)

Three peptide chains

One α-chain and two β-chains

Two α-chains and one β-chain

What is the half-life of thyroid-stimulating hormone in the plasma?

60 minutes (correct)

180 minutes

30 minutes

120 minutes

Which action is NOT a result of thyroid-stimulating hormone activity?

Stimulation of glucose uptake in muscle tissues (correct)

Positive regulation of iodide pump in follicular cells

Enhancement of thyroglobulin secretion into follicles

Increase in the size and secretory activity of follicular cells

How does TSH primarily exert its effects on target cells?

Through cyclic AMP mechanism Signup and view all the answers

Which component is primarily responsible for the storage of thyroid hormones within the follicles of the thyroid gland?

Thyroglobulin Signup and view all the answers

What role does the hypothalamus play in regulating thyroid hormone secretion?

It controls TSH secretion through thyrotropic-releasing hormone (TRH). Signup and view all the answers

What feedback mechanism regulates the secretion of thyroid hormones?

Negative feedback control Signup and view all the answers

How does the weight of the thyroid gland generally differ between genders?

It is larger in females than in males. Signup and view all the answers

Which tissue does NOT show an increase in metabolic activity due to thyroxine?

Brain Signup and view all the answers

Which of the following hormones is primarily responsible for controlling the secretion of thyroid hormones?

Thyroid-stimulating hormone Signup and view all the answers

In hyperthyroidism, how much does the basal metabolic rate increase compared to normal levels?

60% to 100% Signup and view all the answers

What is the main function of the colloidal substance within the thyroid gland?

To store thyroglobulin and thyroid hormones Signup and view all the answers

In which stage does the function of the thyroid gland typically increase?

Pregnancy and lactation Signup and view all the answers

What effect does thyroxine generally have on the force of contraction of the heart?

It increases the force of contraction due to enhanced enzymatic activity. Signup and view all the answers

In hyperthyroidism, what can happen to the heart muscle due to excess activity?

It may weaken due to excess activity and protein catabolism. Signup and view all the answers

What is the anatomical structure that connects the two lobes of the thyroid gland?

Isthmus Signup and view all the answers

What is the primary role of iodine in the thyroid gland?

It is necessary for the synthesis of thyroid hormones. Signup and view all the answers

What is cardiac decompensation?

Failure of the heart to maintain adequate circulation. Signup and view all the answers

How does thyroxine affect blood vessels?

It induces vasodilation by increasing metabolic activities. Signup and view all the answers

The hypothalamus and pituitary regulate thyroid function primarily through which mechanism?

Secretion of hormones that stimulate thyroid activity Signup and view all the answers

What is the typical effect of thyroxine on mean arterial blood pressure?

It usually keeps the mean arterial pressure about normal. Signup and view all the answers

What are the effects of thyroxine on the respiratory rate?

It increases the rate and force of respiration indirectly. Signup and view all the answers

What digestive effects does thyroxine typically induce in the gastrointestinal tract?

It generally increases appetite and GI tract movement. Signup and view all the answers

Why is thyroxine essential for the central nervous system?

It promotes the development of synapses and myelination of axons. Signup and view all the answers

What is the primary reason T4 has a longer duration of action than T3?

T4 binds more tightly to plasma proteins. Signup and view all the answers

Which hormone forms the majority of thyroid hormone secretion?

Tetraiodothyronine (T4) Signup and view all the answers

How much iodine is required weekly to synthesize normal quantities of thyroid hormones?

1 mg Signup and view all the answers

What percentage of total thyroid secretion does T3 represent?

About 7-10% Signup and view all the answers

Which cells synthesize and secrete thyroglobulin?

Follicular cells Signup and view all the answers

What is significant about T3's affinity for plasma proteins?

It binds very loosely, leading to quicker release. Signup and view all the answers

What essential dietary component is iodized in common table salt to prevent deficiency?

Iodine Signup and view all the answers

What mechanism is responsible for transporting iodide from blood into follicular cells?

Sodium-iodide symporter Signup and view all the answers

What happens to the concentration of iodide in the thyroid gland during hyperactivity?

It increases to 200 times the concentration in blood. Signup and view all the answers

Which enzyme is crucial for the oxidation of iodide to elementary iodine?

Thyroid peroxidase Signup and view all the answers

What is the term for the process where iodine binds to thyroglobulin within the follicular cavity?

Organification Signup and view all the answers

Which of the following is NOT a product of iodinated tyrosine coupling reactions?

Monoiodotyrosine (MIT) Signup and view all the answers

What is the role of iodinase in the synthesis of thyroid hormones?

To accelerate the iodination process Signup and view all the answers

What configurations result from the coupling of iodotyrosine residues?

Coupling can yield various thyroid hormones. Signup and view all the answers

How are thyroid hormones stored after synthesis?

As vesicles within thyroglobulin Signup and view all the answers

What is the primary effect of hypersecretion of thyroxine on body temperature?

Increase in body temperature and excess sweating Signup and view all the answers

How does thyroxine influence growth in children?

Insufficient thyroxine leads to stunted growth and brain maturation Signup and view all the answers

What are the effects of increased thyroxine secretion on body weight?

It results in a decrease in body weight and fat storage Signup and view all the answers

What role does thyroxine play in erythropoiesis?

It accelerates erythropoietic activity and increases blood volume Signup and view all the answers

What is an important clinical indicator for diagnosing hypothyroidism and hyperthyroidism?

Heart rate changes due to thyroxine Signup and view all the answers

What happens to children with inadequate levels of thyroid hormone at birth?

They will be mentally deficient throughout life if untreated Signup and view all the answers

What effect does thyroid hormone have on metabolic processes in the body?

Accelerates cellular metabolic processes Signup and view all the answers

What is the impact of thyroid hormone on the growth of bones in hyperthyroid children?

It promotes excessive skeletal growth with early epiphyseal closure Signup and view all the answers

Study Notes

Physiology of Thyroid Gland

The thyroid gland is an endocrine gland located at the base of the neck, consisting of two lobes connected by an isthmus
It develops from an evagination of the floor of the pharynx
It weighs approximately 20-40 grams in adults, larger in females
Thyroid function varies across the menstrual cycle, increasing during pregnancy and lactation, and decreasing during menopause
Iodine is crucial for thyroid hormone synthesis and transport, controlled by mechanisms in the gland
Thyroglobulin, a large glycoprotein, is the major component of the colloid within thyroid follicles
Thyroid hormones, T3 and T4 (thyroxine), are synthesized and stored in the follicles
T3 is significantly more potent than T4. T4 has a longer half-life than T3
Thyroid hormones are crucial for metabolic rate, growth, and development

Histology of Thyroid Gland

Thyroid tissue is composed of closed follicles
Follicular cells line the follicles and secrete thyroglobulin
Colloid, a substance secreted by follicular cells, is contained in the follicles' cavities
Parafollicular cells are interspersed between follicles and release calcitonin

Chemistry of Thyroid Hormones

T3 and T4 are iodine-containing derivatives of tyrosine
T3 is four times more potent than T4
T4 has a longer half-life
Thyroid hormones bind to plasma proteins, influencing their release rate
Total T3 levels are around 0.15µg/dL
Total T4 levels are around 8µg/dL

Synthesis of Thyroid Hormones (6 steps)

Thyroglobulin synthesis: Endoplasmic reticulum and Golgi apparatus synthesize thyroglobulin, a large glycoprotein containing tyrosine
Iodide trapping: Active transport of iodide into follicular cells against electrochemical gradient
Iodide oxidation: Iodide is oxidized to iodine by thyroid peroxidase, a crucial step
Iodine transport into the follicle: Iodine is transported from the follicular cells into the follicular cavity by an iodide-chloride pump (pendrin)
Iodination of tyrosine: Iodine combines with tyrosine residues in thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
Coupling reactions: MIT and DIT residues couple to form T3 and T4 within thyroglobulin

Stages of Thyroid Hormone Synthesis

Thyroid hormones remain within thyroglobulin until needed.
During release, colloid is internalized by thyrocytes via endocytosis
Peptide bonds within thyroglobulin are hydrolysed, releasing T3 and T4

Thyroid Hormone Transport and Metabolism

Thyroid hormones are transported by proteins in the blood (thyroxine-binding globulin, prealbumin, albumin)
Most T4 is converted to T3 in peripheral tissues, making T3 the more active form
T4 has a longer half-life than T3
Degradation occurs in muscles, liver, and kidneys through deiodination

Regulation of Thyroid Hormone Secretion

Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates the release of thyroid-stimulating hormone (TSH) from the anterior pituitary
Thyroid hormones exert negative feedback control on the release of TRH and TSH, maintaining homeostasis
TSH is a crucial factor in regulating thyroid hormone synthesis and release

Function of Thyroid Hormones

Basal Metabolic Rate (BMR): Thyroid hormones increase metabolic activity in most tissues
Protein Metabolism: Thyroid hormones increase protein synthesis and breakdown
Carbohydrate Metabolism: Thyroid hormones increase glucose uptake and glycogen breakdown
Fat Metabolism: Thyroid hormones mobilize fat, decreasing storage and increasing free fatty acid levels
Growth and Development: Essential for normal development, especially of the brain
Body Temperature: Thyroid hormones increase body temperature, leading to heat production
Cardiovascular System: Thyroid hormones increase heart rate and force of contraction
Sleep: Normal thyroxine levels are necessary to maintain normal sleep patterns
Sexual Function: Thyroid hormones are crucial for normal sexual function in both men and women
Other Endocrine Glands: Thyroxine increases the demand for secretion from other endocrine glands
Gastrointestinal Tract: Thyroxine increases secretions and movements of the gastrointestinal tract.
Central Nervous System: Essential for normal development and function of the central nervous system (CNS).
Skeletal Muscle: Essential for normal activity of skeletal muscles
Respiration: Thyroid hormones increase respiration indirectly by increasing the body's demand for oxygen and carbon dioxide

Thyroid Function Tests

T3 and T4 levels, TSH levels

Interpretation of Thyroid Function Tests (TSH, T4, T3)

Results vary depending on abnormalities of the thyroid gland (primary, subclinical, central hypo/hyperthyroidism)

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Description

This quiz covers the essential aspects of the thyroid gland's physiology and histology. It delves into the structure, function, hormone synthesis, and significance of iodide and thyroglobulin. Ideal for those studying human anatomy and endocrinology.