Thyroid Physiology and Hormone Regulation Quiz

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Questions and Answers

Why should free T4 levels be assessed before a morning dose of levothyroxine?

To avoid skewed results that may result from recent levothyroxine intake.

What makes Total T4 levels unreliable for assessing thyroid function?

Total T4 is affected by transport protein levels, which can vary in different conditions.

Why is Total T3, not the best test for primary hypothyroidism?

In primary hypothyroidism, increased conversion of T4 to T3 by MDI can preserve T3 levels, making it an insensitive measure.

What is a major limitation of free T3 assays?

Most commercial assays for free T3 are erratic, limiting their clinical utility. Signup and view all the answers

What is the most common congenital endocrine disorder in childhood?

Congenital hypothyroidism. Signup and view all the answers

What is the primary role of thyroid hormones in the human body?

Thyroid hormones are important for regulating growth, development, thermogenesis, and metabolism. Signup and view all the answers

Describe the physical location of the thyroid gland in the neck.

The thyroid gland is located below the thyroid cartilage in the middle of the neck. Signup and view all the answers

What are the two transcription factors that help regulate thyroid gland development?

The two transcription factors are TTF-1 and TTF-2. Signup and view all the answers

Which cells within the thyroid gland synthesize thyroid hormones, and what is their role regarding iodine?

Follicular cells synthesize thyroid hormones and act as iodine concentrators. Signup and view all the answers

What is pendrin's role in the synthesis of thyroid hormones?

Pendrin transports inorganic iodine to the basolateral surface of follicular cells. Signup and view all the answers

What is the enzyme responsible for organifying iodine?

The enzymes are thyroid peroxidase (TPO) and Dual oxidase 2 (Duox2). Signup and view all the answers

How are T3 and T4 formed in relation to MIT and DIT?

Two molecules of DIT combine to form T4, and one DIT combines with one MIT to form T3. Signup and view all the answers

What are the percentages of circulating T4 and T3 directly produced by the thyroid gland?

The thyroid gland produces 90% of circulating T4 and 20-30% of T3. Signup and view all the answers

What are the three main features of thyroid dysgenesis?

Absent thyroid on ultrasound, no radiotracer uptake, and undetectable thyroglobulin levels. Signup and view all the answers

What is the key identifying feature of a TSH receptor defect, differentiating it from dysgenesis?

Detectable thyroglobulin levels. Signup and view all the answers

Dyshormonogenesis is typically associated with what two clinical features?

Milder hypothyroidism and goiter. Signup and view all the answers

Name two common causes for transient hypothyroidism in newborns.

Iodine deficiency and iodine excess. Signup and view all the answers

What maternal factor can cause transient hypothyroidism in a newborn?

Maternal TSH receptor antibody. Signup and view all the answers

In central hypothyroidism, are TSH levels high or low?

Low or inappropriately normal. Signup and view all the answers

How can you differentiate between a secondary and tertiary cause of central hypothyroidism?

A TRH stimulation test. Signup and view all the answers

Name one genetic mutation that can cause central hypothyroidism.

TSH gene mutation Signup and view all the answers

What is the most common cause of primary congenital hypothyroidism?

Thyroid dysgenesis Signup and view all the answers

What are the two subtypes of thyroid dysgenesis?

Ectopic thyroid & dyshormonogenesis Signup and view all the answers

Besides a defective thyroid, what other cause can lead to primary congenital hypothyroidism?

Iodine deficiency. Signup and view all the answers

How is secondary congenital hypothyroidism primarily diagnosed?

TRH stimulation test Signup and view all the answers

What lab findings are indicative of primary hypothyroidism?

Elevated TSH with low/normal T4 levels Signup and view all the answers

Why do patients with ectopic thyroid typically have reduced thyroglobulin levels?

Decreased functional tissue Signup and view all the answers

What imaging modality is most useful in identifying an ectopic thyroid?

Radionuclide scan Signup and view all the answers

What maternal factor can lead to transient congenital hypothyroidism?

Maternal TRAb Signup and view all the answers

List three symptoms of congenital hypothyroidism in newborns.

Decreased activity/hypotonia, poor feeding/weight gain, and large head and tongue. Signup and view all the answers

What are three criteria, according to the text, for a disorder to qualify for neonatal screening?

The disorder should be common, have a significant impact, and have a lag time. Signup and view all the answers

Why are thyroid functions assessed after 72 hours of life in newborns?

To avoid the interference of the postnatal TRH surge. Signup and view all the answers

What is the preferred initial test for congenital hypothyroidism, according to the text?

TSH is the preferred initial test. Signup and view all the answers

Beyond 14 days, what permanent impact is associated with congenital hypothyroidism if left untreated?

Severe mental retardation. Signup and view all the answers

What condition is indicated when a patient has elevated T3, T4, and TSH levels, despite the presence of peripheral hypothyroidism?

ØTR beta defect Signup and view all the answers

What enzyme, if unusually active, can cause thyroxine-unresponsive hypothyroidism?

MDI 3 Signup and view all the answers

An MCT8 defect presents with severe fetal-onset hypothyroidism, high T3, and low T4, what is this condition also known as?

Allan-Herndon-Dudley syndrome Signup and view all the answers

What range of TSH levels is characteristic of isolated thyrotropinemia, given normal FT4 levels?

6-20 mU/L Signup and view all the answers

In individuals with Trisomy 21, what is the typical finding in terms of T4 levels and TSH levels?

T4 in the lower normal range with mildly elevated TSH levels Signup and view all the answers

What is the typical TSH level presented in transient hypothyroxinemia of prematurity?

↓ or normal TSH Signup and view all the answers

What clinical picture can dopamine use result in, in relation to thyroid hormones?

Central hypothyroidism Signup and view all the answers

Infants with congenital hypothyroidism are usually born at what stage of gestation?

At term or after term Signup and view all the answers

Study Notes

Thyroid Disorders in Children

Thyroid disorders are a significant concern in children, affecting growth, development, and metabolism.
The thyroid gland is located in the middle of the neck, beneath the thyroid cartilage.
The gland comprises two lobes, an isthmus, and sometimes a pyramidal lobe. These lobes originate from different embryonic sources.
Thyroid hormones (T3 and T4) are crucial for regulating growth, development, metabolism, and body temperature. They are synthesized within the follicular cells of the thyroid gland.
Iodine uptake is the first step in thyroid hormone synthesis. Iodine is transported into the follicular cell by a sodium-iodide transporter.
Inorganic iodine is then transported to the basolateral surface by the protein pendrin, and organified by the enzymes thyroid peroxidase (TPO) and Dual oxidase 2 (Duox2).
Organified iodine is incorporated into thyroglobulin, combining with tyrosine to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
Two DIT molecules fuse to create thyroxine (T4), while one DIT and one MIT combine to form triiodothyronine (T3).
Ninety percent of circulating T4 and 20-30% of T3 is directly produced by the thyroid gland. Remaining T3 is generated from peripheral conversion of T4 by the enzyme monodeiodinase I (MDI).
Free T4 and T3 circulate bound to proteins (TBG, albumin, transthyretin).
T4 is primarily bound to TBG, while T3 binds to all three.
Thyroid hormone metabolism involves the conversion of T4 to T3, and their inactivation to reverse T3 (rT3) and T2 by type 3 MDI in target tissues.
T3 is the active thyroid hormone, entering cells and binding to intracellular receptors to regulate gene expression.
Thyroid hormone production and release in the body is controlled by TSH, secreted by pituitary thyrotropes acting on TSH receptors. This regulates iodine uptake, organification, coupling, and hormone release. Also regulates follicular development.
TSH secretion controlled by the hypothalamic hormone, TRH (thyrotropin-releasing hormone).
Thyroid function assessment in a child includes clinical evaluation and assessment of thyroid function and structure. A full history and examination are important.
Common symptoms of hypothyroidism in children include weakness, lethargy, weight gain, somnolence, facial puffiness, decreased performance, and decreased activity/ hypotonia.
Symptoms of hyperthyroidism in children include weight loss, anxiety, tremors, eye protrusion, neck swelling, and the presence of type 1 diabetes mellitus or celiac disease.
Thyroid examination typically assesses size, mobility, and consistency. The presence of a goiter may indicate further testing.
Thyroid function tests—TSH, T4, and potentially T3—are used to evaluate thyroid dysfunction. Free T4 is a more consistent indicator than total T4.
Congenital hypothyroidism (CH) is the most common congenital endocrine disorder in childhood and is largely preventable. It is defined as thyroid hormone deficiency present at birth, with approximately 1 case per 1500 live births.
Causes of CH include thyroid dysgenesis, dyshormonogenesis, TSH resistance, transient hypothyroidism, and peripheral hypothyroidism.
Secondary (central) hypothyroidism arises from pituitary or hypothalamic abnormalities, characterized by low/inappropriately normal TSH with low free T4. Causes include CNS insult, tumor, radiation, or trauma.
Primary hypothyroidism is diagnosed by the presence of elevated TSH levels with low or normal T4 levels. Common causes include ectopic thyroid, genetic defects, or dyshormonogenesis.
Treatment for hypothyroidism typically involves levothyroxine replacement therapy, with careful monitoring of free T4 and TSH levels. Dose adjustment is often necessary.

Acquired Hypothyroidism

Subclinical hypothyroidism is frequently experienced by adolescents.
Causes of acquired hypothyroidism include chronic lymphocytic (Hashimoto's) thyroiditis, iodine deficiency, medications and post-radiation or ablative (radioiodine therapy or surgery).
Symptoms, physical examination, and investigations aid in diagnosis.

Thyrotoxicosis in Children

Thyrotoxicosis, while rare in children is important to identify. It generally is characterized by an increased production of thyroid hormones, leading to clinical manifestations like weight loss, hyperdefection (often associated with increased urination-polyuria) , tremors, anxiety, irritability, and worsening school performance.
Causes of thyrotoxicosis include TSH receptor activating mutations, toxic thyroid nodules, toxic multinodular goiter, subacute lymphocytic (granulomatous) thyroiditis, and acute thyroiditis.
Graves' disease is a common cause of thyrotoxicosis, predominantly in females affecting most children who present with symptoms.
Diagnosing thyrotoxicosis involves identifying the cause, evaluating complications, and considering a comprehensive clinical and laboratory evaluation, including thyroid function tests (TFTs), erythrocyte sedimentation rate (ESR), and potentially antibodies (e.g., TSH receptor antibodies).
Management strategies for thyrotoxicosis may include antithyroid drugs, radioactive iodine, or surgery, adjusting the strategy based on child characteristics.
Monitoring is critical; after treatment the thyroid profile needs to monitored for 6 weeks and then monthly to ensure proper response to the treatment and prevent complications.

Congenital Hypothyroidism (CH) Assessment

Evaluation of newborns for hypothyroidism (neonatal screening) is essential.
Screening for CH typically involves measuring TSH levels—high TSH levels suggest a need for further evaluation.
Typical presentation involves certain signs and symptoms and physical characteristics if diagnosed at an early age or later ages.
Confirmation of the diagnosis involves assessing underlying factors. If no improvement or the disorder is prolonged, confirmatory medical procedures may be required.

Monitoring of Thyroid Function

Monitoring of thyroid hormone levels (free T4 and TSH) is vital throughout treatment to maintain appropriate levels.
Adjustments to medication doses are typically needed to maintain the proper target levels of free thyroxine. This requires accurate tracking and frequent dosage adjustments.
Persistent monitoring of thyroid function tests is essential after treatment, with appropriate dosage adjustments.

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Description

This quiz focuses on key concepts related to thyroid physiology, hormone synthesis, and the assessment of thyroid function. It covers various tests, the role of thyroid hormones, and developmental factors related to the thyroid gland. Assess your knowledge about this critical endocrine system.