THYROID GLAND (LAB)

lower anterior neck

Thyroid gland is positioned in the _______, shaped like a butterfly

isthmus

Thyroid gland is made up of two lobes, resting on each side of the trachea, bridged by the ________

Thyroid gland

Responsible for the production of thyroid hormone and calcitonin (parafollicular cells)

o Body metabolism

o Neurologic development

o Tissue growth

o Heat production

o Control of oxygen consumption

o Energy conservation

Body functions of Thyroid hormone

Parathyroid gland

- posterior the thyroid gland

- Regulate serum calcium levels

Iodine

- found in seafood, dairy products, and vitamins

- Key determinant in thyroid function

Hypothalamic Pituitary Thyroid axis

neuroendocrine system that regulates the production and secretion of thyroid hormones

o Thyroxine-binding globulin (TBG

o Thyroxine-binding prealbumin (TBPA) - Transthyretin

o Albumin

Majority of the thyroid hormones are bound to carrier proteins:

anterior pituitary gland

Origin of Thyroid Stimulating Hormone

Protein

Classification of TSH

Thyrotropin-releasing hormone (TRH) and Thyroid hormones

Regulators of TSH

Thyroid Stimulating Hormone

The most useful test in assessing thyroid function

FIRST-GENERATION TSH

based on radioimmunoassay but with poor sensitivity

- Cross-reacts with HCG

SECOND-GENERATION TSH

- (immunometric assays) effectively screen for hyperthyroidism

THIRD-GENERATION TSH

(chemiluminometric assays) give fewer false-negative results and more accurately distinguish between euthyroidism and hyperthyroidism

THIRD-GENERATION ULTRASENSITIVE TSH (s-TSH)

preferred method for monitoring and adjusting thyroid hormone replacement therapy and screening for abnormal thyroid hormone production in the clinical setting

FOURTH-GENERATION TSH ASSAY

used mainly for research purposes

10-fold

Fourth-Generation TSH assay provides ______ increase in sensitivity compared to third-generation assays

thyroxine (T4) / tetraiodothyronine

• Usually measured by chemiluminometric assay or similar immunometric technique

• Principal secretory product of the thyroid gland

• Its plasma concentration is a good indicator of the thyroid secretory rate

• In the peripheral tissue, T4 is converted to T3

0.4-4.0 mIU/L

Reference range of Thyroxine

20

T3 population:

o __% - produced by thyroid gland

80

T3 population:

o __% - derived from the deiodination of T4

Triiodothyronine (T3)

• More biologically active thyroid hormone

• More potent that T4 has shorter half-life

Total T3

composed of both bound and free fractions

Carrier protein

Majority of total T3 are bound to ________

80-200ng/dL

Reference range of Triiodothyronine (T3)

Free Thyroxine (FT4)

• Biologically active fraction of T4

• Used as a second-line test in the evaluation of suspected thyroid disorders

hyperthyroidism, excess thyroid hormone replacement

Increased values of Free Thyroxine (FT4)

o Suggest _____________

o may also be caused by __________

hypothyroidism

Decreased values of Free Thyroxine (FT4) suggest ____________

abnormal TBBG and/or albumin

Free thyroxine (FT4) Levels may be affected by _________ and/or ________

thyroid autoantibodies, biotin, and human anti-mouse antibodies (HAMA)

Interfering substances of Free Thyroxine (FT4)

0.9-1.7 ng/dL

Reference range of Free Thyroxine

Bound

Approx.. 99.7% of total T3 is ______

Free Triiodothyronine (FT3)

• Biologically active form

• Measured less frequently - due to low fraction

• Third-level test of thyroid function

• May be required to evaluate clinically euthyroid patients (with abnormal thyroid binding proteins)

hyperthyroidism, excess thyroid hormone

Increased values of Free Triiodothyronine (FT3)

- commonly associated with ___________ or with ___________

Thyroglobulin

- protein synthesized and secreted exclusively by thyroid follicular cells

- Proof of the presence of thyroid tissue - ideal tumor marker and post-treatment thyroid cancer surveillance

test

o Immunoenzymatic (sandwich)

o ELISA

o Immunoradiometric assay (IRMA)

o Immunochemiluminescent assay (ICMA)

Thyroglobulin can be measured by the following:

True

True or False

The accuracy of the thyroglobulin assay is primarily DEPENDENT on the SPECIFICITY of the antibody used and the absence of antithyroglobulin

Thyroid autoimmunity

antibodies are directed at thyroid tissue

Thyroglobulin autoantibodies

- antibodies against thyroglobulin

- Most commonly associated with Hashimoto's thyroiditis

- 35-60% of patients with hypothyroidism due to autoimmune disease have anti-Tg autoantibodies

<4.0 IU/mL

Reference range of Thyroglobulin autoantibodies

Thyroperoxidase Antibody

- most sensitive test for detecting autoimmune thyroid diseases

- Used as a diagnostic tool in deciding whether to treat a patient with subclinical

hypothyroidism

<9.0IU/mL

Reference range of Thyroperoxidase antibody

Thyrotropin Receptor Antibody (TRAb)

- most closely associated with disease pathogenesis

- Long-acting thyroid stimulator (LATS)

- Thyroid-stimulating immunoglobulins (TSI) - IgG; can cross placenta = neonatal thyrotoxicosis

>1.75 IU/L

Reference range of Thyrotropin Receptor Antibody (TRAb)

1. Nuclear Medicine evaluation

2. Thyroid Ultrasound

3. FNAB

Other Tools for Thyroid Evaluation

Radioactive iodine

o useful in assessing the metabolic activity of thyroid tissue

o Assists in the evaluation and treatment of thyroid cancer

Radioactive iodine uptake (RAIU)

percentage of the dose taken up by the thyroid gland

Hight uptake of RAIU

gland is metabolically active and producing significant amounts of thyroid hormone

Low uptake of RAIU

thyroid gland is inactive

Thyroid ultrasound

• A noninvasive procedure that uses high frequency sound waves to produce image

• Assessment of thyroid anatomy and characterization of palpable thyroid abnormalities

• Also able to detect non-palpable thyroid nodules

FNAB (Fine Needle Aspirate Biopsy)

• OFTEN first step and most accurate tool in the evaluation of thyroid nodules in the absence of

hyperthyroidism

• Commonly used with ultrasound-guided imaging

• Allows prompt identification and treatment of thyroid malignancies

• Cells are aspirated for cytologic evaluation and reported by the pathologist (Bethesda System for

Reporting Thyroid Cytopathology)

o Nondiagnostic/unsatisfactory

o Benign

o Atypia/follicular lesion of undetermined significance

o Follicular neoplasm/suspicious for follicular neoplasm o Suspicious for malignancy

o Malignancy

Hypothyroidism

defined as low FT4 with normal or high TSH

• One of the most common disorders of the thyroid gland

• Symptoms:

o cold intolerance

o Fatigue

o Dry skin

o Constipation

o Hoarseness

o Dyspnea on exertion

o Cognitive dysfunction

o Hair loss and weight gain

Primary hypothyroidism

thyroid gland dysfunction

Secondary Hypothyroidism

Pituitary dysfunction

Tertiary hypothyroidism

Hypothalamic dysfunction

Hashimoto's Thyroiditis

- most common cause of hypothyroidism

o Autoimmune disease targeting the thyroid gland

o Often associated with an enlarged gland (goiter)

Thyrotoxicosis

- group of syndromes caused by high levels of free thyroid hormones in the circulation

• can be the result excessive thyroid hormone ingestion, leakage of stored thyroid hormone from thyroid

follicles, or excessive thyroid gland production of thyroid hormone

Hyperthyroidism

refers to an excess of circulating thyroid hormones

Grave's disease

- most common cause of hyperthyroidism

o Autoimmune disease in which antibodies are produced that activate the TSHR

o Features: thyrotoxicosis, goiter, ophthalmopathy, dermopathy

o Strong familial disposition

o Women are more likely to develop it than men

o LaboratoryResults: ↑FT4and/orT3 level with↓ or undetectable TSH

> TSI and TRAb = usually +

> Elevated RAIU

Toxic Adenoma and Multinodular Goiter

o Caused by autonomously functioning thyroid tissue

o Neither TSH nor TSHR-stimulating immunoglobulin is required to stimulate thyroid hormone production

Amiodarone-Induced Thyroid disease

- fat-soluble drug with a long half-life that is used to treat

cardiac arrythmias

o Interferes with normal thyroid function

o Also block T4 to T3 conversion

o Patients taking this medication for a long period of time can develop this hyperthyroidism

Subacute Thyroiditis

Conditions associated with a thyrotoxic phase when thyroid hormone is leaking into the circulation, a hypothyroid phase when the thyroid is repairing itself, and a euthyroid phase

when the gland is repaired

Post-partum thyroiditis

- most common form of subacute thyroiditis

- Strongly associated with the presence of TPO antibodies and chronic lymphocytic thyroiditis

- Patients may experience thyrotoxicosis

Painless thyroiditis

similar characteristics with post-partum thyroiditis but with an absence of pregnancy

Painful thyroiditis

also called subacute granulomatous / subacute nonsuppurative thyroiditis / De Quervain's thyroiditis

- Characterized by: neck pain, low-grade fever, myalgia, tender diPuse goiter

- Negative TPOAb

- ↑ ESR and thyroglobulin

Nonthyroidal illness

o refers to abnormalities in the thyroid function tests without thyroid dysfunction o AKA euthyroid sick syndrome

o Laboratory pattern: normal or low TSH, low T3 and low FT4

o Illness decreases 5'-monodeiodinase activity = less conversion of T4 to T3

Thyroid nodules

o Only 6-9% of nonpalpable nodules proved to be thyroid cancer

FNAB

routine procedure to distinguish nodules