Anatomy of the Thyroid Gland

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Questions and Answers

What is the anatomical relationship between the thyroid gland and the trachea?

The thyroid gland is anterior to the second to fourth tracheal rings.

What is the function of parafollicular cells in the thyroid gland?

Parafollicular cells produce calcitonin, which inhibits bone resorption.

What is the role of TSH in the regulation of thyroid hormone synthesis and secretion?

TSH stimulates thyroid cell growth, iodide uptake, and thyroid hormone synthesis and secretion.

What is the source of T4 and T3 in the body?

The thyroid gland is the exclusive source of T4, while approximately 80% of T3 is from peripheral T4 deiodination. Signup and view all the answers

What is the binding pattern of T4 and T3 in the blood?

Most of T4 (99.96%) and T3 (99.6%) are bound to serum proteins, with approximately 70% bound to T4-binding globulin (TBG). Signup and view all the answers

What is the negative feedback mechanism that regulates thyroid hormone synthesis and secretion?

T4 and T3 exert negative feedback on the hypothalamus and pituitary. Signup and view all the answers

What is the biological significance of free T4 and T3?

Only free T4 and T3 are biologically available, with T4 serving as a prohormone and T3 binding to cellular nuclear receptors to affect gene transcription in target tissues. Signup and view all the answers

What is the specific effect of T3 on the cardiovascular system?

T3 has positive cardiac inotropic and chronotropic effects, enhances myocardial adrenergic sensitivity, and increases myocardial diastolic relaxation. Signup and view all the answers

What percentage of the population has nodules identified on ultrasound?

40% Signup and view all the answers

What is the approximate percentage of thyroid nodules that contain malignancy?

5% Signup and view all the answers

What is the primary laboratory test used to evaluate a thyroid nodule?

Measuring TSH Signup and view all the answers

What is the purpose of a thyroid scan in evaluating a thyroid nodule?

To determine the percentage of radioactive iodine uptake by the thyroid and obtain an image. Signup and view all the answers

What is the significance of a 'hot' nodule in thyroid evaluation?

Hot nodules have a low risk of malignancy and do not require fine-needle aspiration biopsy. Signup and view all the answers

According to the American Thyroid Association guidelines, how are thyroid nodules classified?

Based on echogenicity, solidity or cystic nature, and features of malignancy. Signup and view all the answers

What percentage of biopsied nodules have evidence of malignancy?

5-10% Signup and view all the answers

What is the recommended approach for performing thyroid FNAB?

Under ultrasound guidance whenever possible due to improved accuracy. Signup and view all the answers

What measurement is indicated for all thyroid nodules and why is it important?

TSH measurement. Because structural abnormalities in the thyroid lead to increased risk for thyroid dysfunction. Signup and view all the answers

How often should repeat ultrasonography be performed for high-suspicion nodules?

In 6 to 12 months. Signup and view all the answers

When is repeat FNAB indicated?

For high-suspicion nodules, nodules with concerning new sonographic findings, and intermediate- or low-suspicion nodules that increase in size. Signup and view all the answers

What is the initial evaluation step for a thyroid nodule?

Measuring serum thyroid-stimulating hormone (TSH) level. Signup and view all the answers

What is the most common cause of goiter worldwide?

Iodine deficiency. Signup and view all the answers

What symptoms may indicate compression in patients with goiter?

Dyspnea, cough, dysphagia, and voice changes. Signup and view all the answers

What is the first laboratory step to assess goiter?

Measuring TSH level. Signup and view all the answers

What is multinodular goiter (MNG)?

The most common cause of thyroid enlargement in older adults in the United States. Signup and view all the answers

What should patients with goiter be questioned about?

Iodine intake, rate of change in size, and thyroid cancer risk factors. Signup and view all the answers

What is the indication for thyroid ultrasonography in patients with goiter?

Palpable thyroid nodules, gland asymmetry, large goiter, rapid growth, or compressive symptoms. Signup and view all the answers

What is the most sensitive test of thyroid function and when is it sufficient as an initial test?

Serum thyroid-stimulating hormone, sufficient as an initial test unless central hypothyroidism is suspected Signup and view all the answers

What is the difference between hyperthyroidism and thyrotoxicosis?

Thyrotoxicosis is high levels of circulating thyroid hormones from any cause, while hyperthyroidism is thyrotoxicosis caused by excessive endogenous thyroid hormone production. Signup and view all the answers

What is the significance of a low TSH and elevated free T4 and/or total T3 in diagnosing thyrotoxicosis?

It confirms the diagnosis of thyrotoxicosis. Signup and view all the answers

What is the purpose of thyroid scintigraphy with RAIU in the diagnosis of hyperthyroidism?

To verify the cause of hyperthyroidism. Signup and view all the answers

What is the significance of lid lag in thyrotoxicosis?

It is a sign of increased adrenergic tone, which can be seen in thyrotoxicosis of any cause. Signup and view all the answers

What is the prevalence of hyperthyroidism in the United States?

1.3% Signup and view all the answers

What additional test is indicated in patients with suspected substernal extension of the thyroid gland?

CT or MRI of the neck and chest Signup and view all the answers

Why should administration of iodinated contrast be avoided in thyroid imaging?

To avoid precipitating iodine-induced hyperthyroidism (Jod-Basedow phenomenon) Signup and view all the answers

What is the most common cause of diffuse goiter in the United States?

Autoimmune thyroid disease associated with thyroid dysfunction (Hashimoto thyroiditis and Graves disease) Signup and view all the answers

When is thyroid surgery considered in patients with diffuse goiter?

In the setting of significant compression Signup and view all the answers

What is the incidence rate of thyroid cancer in the United States?

13.9 per 100,000 people per year Signup and view all the answers

What type of thyroid carcinoma commonly spreads to cervical lymph nodes?

Papillary thyroid carcinoma (PTC) Signup and view all the answers

What is a risk factor for thyroid cancer?

A history of ionizing radiation exposure Signup and view all the answers

What is the mainstay of thyroid cancer treatment?

Surgery Signup and view all the answers

What is the diagnostic test of choice for thyroid cancer?

Fine-needle aspiration biopsy (FNAB) Signup and view all the answers

What is a symptom that suggests malignancy in a thyroid nodule?

Rapid nodule growth, a hard fixed nodule, dysphagia, hoarseness, and cervical lymphadenopathy Signup and view all the answers

When is hemithyroidectomy acceptable for thyroid cancer?

For unilateral differentiated thyroid cancers with nodules 1 to 4 cm if locoregional spread is not suspected Signup and view all the answers

What is a potential risk of thyroid surgery?

Hypocalcemia from parathyroid injury or removal and difficulty breathing or voice changes from recurrent laryngeal nerve injury. Signup and view all the answers

What is the definition of low-risk PTC?

Confined to the thyroid, completely resected, not metastasized, and without aggressive pathologic features. Signup and view all the answers

Why is postoperative 131I considered for thyroid remnant ablation and adjuvant therapy?

For differentiated thyroid cancer with an intermediate to high risk of recurrence, such as with extrathyroidal extension, lymph node involvement, vascular invasion, poorly differentiated or more aggressive histology, or metastatic disease. Signup and view all the answers

What is the purpose of whole-body scanning after 131I therapy?

To identify areas of 131I uptake corresponding to metastatic disease. Signup and view all the answers

What is the significance of persistent thyroglobulin (Tg) levels after initial cancer treatment?

They suggest recurrent or persistent disease. Signup and view all the answers

Why is thyroid ultrasonography repeated in routine thyroid cancer surveillance?

To monitor for residual or recurrent thyroid cancer. Signup and view all the answers

What is the purpose of TSH suppression with daily levothyroxine in differentiated thyroid cancer treatment?

To suppress the TSH with the target TSH appropriate for the risk of recurrence. Signup and view all the answers

How is metastatic thyroid cancer managed?

With active surveillance, additional surgery, or 131I therapy, as well as external beam radiation therapy and/or chemotherapy (tyrosine kinase inhibitors). Signup and view all the answers

What is the characteristic presentation of anaplastic thyroid cancer?

A rapidly enlarging neck mass, which may be unresectable at the time of diagnosis. Signup and view all the answers

What is the genetic association with medullary thyroid cancer?

Germline RET oncogene mutations occur with familial medullary thyroid cancer and MEN 2A and 2B. Signup and view all the answers

What is the primary goal of TSH level management in patients with medullary thyroid cancer?

to achieve a TSH level within the reference range Signup and view all the answers

What is the preferred treatment for medullary thyroid cancer?

total thyroidectomy and central neck lymph node dissection Signup and view all the answers

Why should MEN be ruled out before surgery in patients with medullary thyroid cancer?

to screen for pheochromocytoma, which should be treated first if present Signup and view all the answers

What laboratory tests are part of routine cancer surveillance in patients with medullary thyroid cancer?

serum calcitonin, serum carcinoembryonic antigen levels, and neck ultrasonography Signup and view all the answers

What is the most sensitive thyroid function test in patients with normal pituitary function?

Serum TSH Signup and view all the answers

What is the significance of a suppressed TSH level in patients with thyroid dysfunction?

it may indicate overt or subclinical hyperthyroidism Signup and view all the answers

What is the effect of biotin supplementation on thyroid function tests?

it can cause falsely high free T4, free T3, total T4, and total T3, and falsely low TSH levels Signup and view all the answers

In what settings is measurement of T3 recommended in patients with hyperthyroidism?

evaluation of thyrotoxicosis, assessment of hyperthyroidism severity, and differentiation of hyperthyroidism from destructive thyroiditis Signup and view all the answers

Why is levothyroxine used in patients with differentiated thyroid cancer?

to treat postoperative hypothyroidism and suppress thyroid-stimulating hormone Signup and view all the answers

What is the preferred test for measuring free T4 in patients with thyroid dysfunction?

immunometric assays Signup and view all the answers

Study Notes

Thyroid Anatomy and Physiology

The thyroid gland consists of right and left lobes connected by a median isthmus, located in the neck between the sternal notch and thyroid cartilage.
The parathyroids are located behind the thyroid lobes, with two superior and two inferior parathyroids.
The recurrent laryngeal nerves course behind the thyroid gland.
The thyroid gland contains parafollicular cells (C cells) and follicular cells.
Parafollicular cells produce calcitonin, which inhibits bone resorption and plays a minor role in bone physiology.
Follicular cells produce thyroid hormones thyroxine (T4) and triiodothyronine (T3).

Thyroid Hormone Regulation

The hypothalamic-pituitary-thyroid axis regulates thyroid hormone synthesis and secretion.
Hypothalamic thyrotropin-releasing hormone (TRH) triggers the pulsatile release of thyroid-stimulating hormone (TSH) from the anterior pituitary.
TSH stimulates thyroid cell growth, iodide uptake, and thyroid hormone synthesis and secretion.
T4 and T3 exert negative feedback on the hypothalamus and pituitary, which further moderates hormone synthesis.
The thyroid gland is the exclusive source of T4, while approximately 80% of T3 is from peripheral T4 deiodination, primarily in the liver and kidney.
Most T4 (99.96%) and T3 (99.6%) are bound to serum proteins, with only free T4 and T3 being biologically available.

Thyroid Function and Hormone Effects

T3 binds with high affinity to cellular nuclear receptors, affecting gene transcription in target tissues.
T3 has positive cardiac inotropic and chronotropic effects, enhances myocardial adrenergic sensitivity, increases myocardial diastolic relaxation, and augments intravascular volume.
T3 also increases gastrointestinal motility, bone turnover, heat generation, and energy expenditure.

Thyroid Examination

The thyroid gland is located in the neck between the sternal notch and thyroid cartilage.
The thyroid gland attaches to the trachea posteriorly and elevates with swallowing and neck extension.
Examination involves both inspection and palpation while the patient swallows liquid with the neck slightly extended.

Thyroid Nodules

Palpable nodules are found in 5% of women and 1% of men.
Approximately 40% of the U.S. population has nodules identified on ultrasound, with an increasing incidence with age.
Causes of thyroid nodules range from benign cysts and inflammatory nodules to malignancies.
The initial laboratory evaluation of a thyroid nodule begins with measuring TSH.
Patients with a suppressed TSH level are evaluated with thyroid scintigraphy, while those with normal or elevated TSH are evaluated with ultrasonography.
Thyroid nodule characteristics and size determine the need for fine-needle aspiration biopsy (FNAB).

Goiter

Goiter is an enlarged thyroid gland, which can be associated with normal thyroid function, hypothyroidism, or hyperthyroidism.
The most common cause of goiter worldwide is iodine deficiency.
Patients with goiter should be questioned about iodine intake, rate of change in size, and thyroid cancer risk factors.
Clinical history should focus on symptoms of thyroid hormone excess or deficiency and compression.
Examination should note the thyroid size, symmetry, and consistency, as well as the presence of nodules or adenopathy and tracheal deviation.

Multinodular Goiter

Multinodular goiter (MNG) is the most common cause of thyroid enlargement in older adults in the United States.
Evaluation includes measurement of TSH and thyroid ultrasonography to evaluate discrete nodules.
The frequency of malignancy in patients with MNG is similar to those with solitary thyroid nodules.
Signs and symptoms of compression or suspected substernal extension require additional testing, such as CT or MRI of the neck and chest.

Thyroid Cancer

Thyroid cancer is diagnosed in 13.9 per 100,000 people per year in the United States.
Thyroid cancer incidence has increased during the past four decades, mainly due to the rise in the diagnosis of small noninvasive cancers.
Risk factors for thyroid cancer include a history of ionizing radiation exposure and a personal or family history of thyroid cancer.
The diagnosis is confirmed by FNAB, and surgery is the mainstay of thyroid cancer treatment.
Postoperative 131I therapy should be considered for thyroid remnant ablation and adjuvant therapy for differentiated thyroid cancer with an intermediate to high risk of recurrence.

Thyroid Function Evaluation

Serum TSH is the most sensitive thyroid function test in patients with normal pituitary function.
If TSH is suppressed, free T4 and total T3 should be assessed to detect overt or subclinical hyperthyroidism.
If TSH is elevated, free T4 should be assessed to detect overt or subclinical hypothyroidism.
Measuring TSH alone as an initial test is sufficient except in suspected central hypothyroidism, for which free T4 measurement is preferred.### Thyroid Function Tests
Accurate in most clinical settings, but may be inaccurate with significant perturbations (familial dysalbuminemic hyperthyroxinemia)
Measuring free T4 by equilibrium dialysis is highly accurate, but expensive and rarely necessary
Biotin supplements can interfere with thyroid function tests, causing falsely high free T4, free T3, total T4, and total T3, and falsely low TSH, mimicking thyrotoxicosis

Thyroid Function and Replacement

Multiple drugs can affect thyroid function and replacement (Table 35)
Patients taking > 5-10 mg/day of biotin should discontinue it 2-5 days before thyroid function testing

T3 Measurement

Not necessary in hypothyroidism unless severe, as normal levels are maintained
Recommended in three settings: evaluating thyrotoxicosis, assessing hyperthyroidism severity, and differentiating hyperthyroidism from destructive thyroiditis
In T3 toxicosis, the T3-to-T4 ratio is often > 20 due to preferential secretion of T3

Key Points

Serum TSH is the most sensitive test of thyroid function and is sufficient as an initial test unless central hypothyroidism is suspected
Triiodothyronine measurement is not necessary in hypothyroidism; normal levels are maintained unless hypothyroidism is severe

Disorders of Thyroid Function

Thyrotoxicosis describes high levels of circulating thyroid hormones (T4 and T3) from any cause
Hyperthyroidism is thyrotoxicosis caused by excessive endogenous thyroid hormone production
Prevalence of hyperthyroidism in the US is 1.3%

Clinical Features and Diagnosis

Thyrotoxicosis may present with nonclassical symptoms and signs in older adults
Lid lag (eyelid retraction) can be seen in thyrotoxicosis of any cause due to increased adrenergic tone
Presence of a diffuse goiter and recently developed proptosis is sufficient for a diagnosis of Graves disease
Thyrotoxicosis is diagnosed with a low TSH and elevated free T4 and/or total T3
Thyroid scintigraphy with RAIU can verify the cause

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Description

This quiz covers the structure and location of the thyroid gland, including its relation to surrounding organs and nerves. It also touches on the potential symptoms of thyroid pathology.