MLAB 2401: Clinical Chemistry Keri Brophy-Martinez

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MLAB 2401 Clinical ChemistryKeri Brophy-Martinez

• Thyroid Gland

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Anatomy

• Largest endocrine gland
• Located in the lower part of the neck, wrapped
  around trachea and larynx
• Composed of two cell types
• Follicular
• Make and secrete thyroxine (T4) and
  triodothyronine (T3)
• Clear
• Secrete calcitonin

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Thyroid Hormone Synthesis

• Thyroid hormone is made up of iodine, primarily,
  so we must get iodine into the cell for thyroid
  function
• Daily intake of iodine should be 150 µg
• If lt 50 µg/day thyroid can not make thyroid
  hormone and leads to a deficiency
• Sources of Iodide
• seafood, dairy products, vitamins and
  iodine-enriched breads

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Synthesis of Thyroid Hormones
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Thyroid Hormones

• Triiodothyronine (T3)
• Thyroxine (T4)
• Calcitonin
• Lowers plasma calcium
• Assists in calcium homeostasis
• Reverse T3 (rT3)
• Produced by the liver
• Inactive form of T3

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Protein Binding of Thyroid Hormones

• Once released into circulation, only small
  amounts of T4 and T3 are unbound and available
  for hormone activity
• Binding Proteins
• Thyroxine-binding globulin (TBG)
• Thyroxine-binding prealbumin (TBPA)
• albumin

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Thyroid Hormone Regulation

• Regulation occurs via a feedback system between
  the thyroid gland, hypothalamus and anterior
  pituitary gland
• Hypothalamus produces TRH
• TRH asks the anterior pituitary to make and
  release TSH
• Free T3 and T4 hormones have a negative feedback
  effect to block TRH

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Actions of Thyroid Hormone

• Tissue growth
• Brain maturation
• Increased heat production
• Increased oxygen comsumption

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• Disorders of the Thyroid Gland

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Terms

• Euthyroid
• Normal thyroid gland function
• Goiter
• Condition where the thyroid gland grows larger
  than normal

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Hypothyroidism

• Symptoms
• Obesity, dry skin, dry lusterless hair,
  sluggishness of body functions and goiter
• Lab features
• Low free T4 level
• Normal to high TSH

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Hypothyroidism
Types Dysfunction Condition
Primary Thyroid gland Hashimotos thyroiditis Treatment for toxic goiter Excessive iodine intake Subacute thyroiditis
Secondary Pituitary gland Hypopituitarism
Tertiary Hypothalamus Hypothalamic
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Thyrotoxicosis

• Causes
• Excessive thyroid hormone ingestion
• Leakage of stored thyroid hormone from thyroid
  follicles
• Excessive production of thyroid hormone
  (hyperthyroidism)

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Thyrotoxicosis

• Signs

• Symptoms

• Tachycardia
• Tremor
• Warm, moist, flushed skin
• Goiter
• Muscle wasting and weakness

• Anxiety, nervousness
• Palpitations
• Weight loss
• Heat intolerance
• Prominence of eyes

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Graves Disease

• Autoimmune Disease
• Antibodies activate TSH receptor
• Familial
• Women more affected
• Lab Features
• High free T4 and/or T3 level
• TSH undetectable

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Hyperthyroidism

• http//www.medicinenet.com/hyperthyroidism_picture
  s_slideshow/article.htm

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Subacute Thyroiditis

• Condition where there is inflammation of the
  thyroid gland, leakage of stored thyroid hormone
  and repair of the gland
• Most common form is postpartum thyroiditis

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Nonthyroidal Illness

• Abnormalities in thyroid function tests of
  hospitalized patients (especially critically ill
  patients)
• Characterized by low total T4, free T4, TSH
• Less T4 is converted to active T3, leading to
  decreased levels of T3 and higher levels of
  reverse T3.
• Central hypothyroidism thyroid hormone-binding
  changes are associated with severe illness.
• Changes may be appropriate adaptations to illness.

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Thyroid Nodules

• Common
• Clinically apparent nodules are present in 6.4
  of adult women 1.5 of adult men.
• Thyroid ultrasound finds unsuspected nodules in
  2045 of women 1725 of men.
• 59 prove to be thyroid cancer.
• Fine-needle aspiration, with cytologic
  examination of aspirate, is used to determine
  need for surgical removal.

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• Assessment of the Thyroid

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Historical Procedures

• Basal Metabolic Rate
• Measured rate of oxygen consumption over a 15
  minute period
• Required careful patient preparation and
  precision of testing
• Radioactive iodine uptake test
• Measured the ability of the thyroid to take up
  and trap iodine
• Rate of absorption is determined by measuring
  increased radioactivity in the thyroid gland

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Historical Procedures

• Protein bound iodine (PBI)
• Used to estimate the amount of thyroid T3 and T4
  by measuring the amount of iodine contained in a
  protein precipitate of serum

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Common Thyroid Function Tests

• TSH
• Total T3 and total T4
• Free T3 and Free T4
• T3 Uptake (for binding proteins)
• Free thyroxine index( estimate of Free T4)
• Serological testing for antibodies to thyroid
  disease

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Thyroid Testing Methods

• Competitive binding techniques
• RIA
• ELISA
• FIA

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Thyroid Antibodies

• Thyroglobulin antibodies
• Anti-thyroglobulin antibodies are mostly IgG and
  do not fix complement
• Titerable using an indirect hemagglutination
  assay
• Found in
• Hashimotos thyroiditis
• Graves disease
• Subacute thyroiditis
• Thyroid cancer

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Thyroid Antibodies

• Microsomal antibodies
• Complement fixing IgG directed against a
  microsomal antigen within follicular cells
• Found in
• Hashimotos disease
• Thyrotoxicosis
• Primary myxedema