MLAB 2401: Clinical Chemistry
Keri Brophy-Martinez
Thyroid Gland
Thyroid Anatomy
• Largest endocrine
gland
• Located in the lower
part of the neck,
wrapped around
trachea
Thyroid Cells
• Composed of two cell
types
– Follicular
• Make and secrete
thyroxine (T4) and
triiodothyronine
(T3)
– Parafollicular/ “C”
cells
• Secrete calcitonin
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 < 50 µg/day: thyroid can not make thyroid hormone
and leads to a deficiency
• Sources of Iodine
– seafood, dairy products, vitamins and iodine-enriched
breads
Synthesis of Thyroid Hormones
Thyroid Hormones
• Triiodothyronine (T3)
– Active form of the hormone
– More potent than T4
• Thyroxine (T4)
– Active form of the hormone
– Serves as a reservoir
• Reverse T3 (rT3)
– Found in the liver, kidney
– Inactive form of T3
– Contributes to T3 circulating pool
• Calcitonin
– Lowers plasma calcium
– Assists in calcium homeostasis
Free Vs. Protein Binding
of Thyroid Hormones
• Once released into circulation, only small
amounts of T4 and T3 are unbound/FREE and
available for hormone activity
• Binding Proteins
– Once bound- lacks biological function
– Affects TOTAL hormone levels
• Thyroxine-binding globulin (TBG)
• Thyroxine-binding prealbumin (TBPA)
• Albumin
Thyroid Hormone Regulation
• Regulation occurs via a
feedback system between
the thyroid gland,
hypothalamus and anterior
pituitary gland
1.
2.
3.
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
Actions of Thyroid Hormone
• Regulation of body
metabolism
– Body temperature
• Increases heat production
– Energy use
– Calorie burning
– Oxygen consumption
• Brain maturation
• Tissue growth
Disorders of the Thyroid Gland
Terms
• Euthyroid
– Normal thyroid gland function
• Goiter
– Condition where the thyroid gland grows larger
than normal
Hypothyroidism
• Symptoms
– Vary depending on degree and cause
– Obesity, dry skin, dry lusterless hair, sluggishness
of body functions and goiter
• Lab features
– Low free T4 level
– Normal to high TSH
Hypothyroidism
Types
Dysfunction
Condition
Primary
Thyroid gland
•Hashimoto’s thyroiditis
•Treatment for toxic goiter
•Excessive iodine intake
•Subacute thyroiditis
Secondary
Pituitary gland/Hypopituitarism
•Adenoma
•Radiation therapy
•Pituitary destruction
Tertiary
Hypothalamus/Hypothalamic
Rare
Thyrotoxicosis
• Causes of:
– Excessive thyroid hormone ingestion
– Leakage of stored thyroid hormone from thyroid
follicles
– Excessive production of thyroid hormone
(hyperthyroidism)
Thyrotoxicosis
Signs
• Tachycardia
• Tremor
• Warm, moist, flushed skin
• Goiter
• Muscle wasting and
weakness
Symptoms
• Anxiety, nervousness
• Palpitations
• Weight loss
• Heat intolerance
• Prominence of eyes
Graves’ Disease
• Autoimmune Disease
– Antibodies activate TSH
receptor
• Familial
• Women more affected
• Lab Features
– High free T4 and/or T3
level
– TSH low-undetectable
Thyroiditis
• Condition where there is inflammation of the
thyroid gland
• Categories include acute, subacute, chronic
Nonthyroidal Illness
• Disorders linked to thyroid hormone excess or
deficiency
• Less T4 is converted to active T3, leading to
decreased levels of T3 and higher levels of
reverse T3.
• Normal T4
• Variable TSH
Assessment of the Thyroid
Thyroid Function Tests
• TSH= Thyroid Stimulating Hormone
– Responsible for directing the thyroid gland to
make more thyroid hormone
– Best test to measure thyroid function
– Inversely related to thyroid status
Thyroid Function Tests
• Serum Total T4 and Total T3
– Represents the total amount of T4 or T3 in the
blood
– The higher the values- whether T4 or T3 indicate
HYPERthyroidism
– The lower the values- whether T4 or T3 indicate
HYPOthyroidism
Thyroid Function Tests
• Free T4/Thyroxine
– Usually ordered if TSH abnormal
– Measure free/unbound T4
– Shows the level of hormone available for uptake &
use
– Elevated in HYPERthyroidism
– Decreased in HYPOthyroidism
Thyroid Function Tests
• Free T3/ Triiodothyronine
– Measure free/unbound T3
– Shows the level of hormone available for uptake &
use
Thyroid FunctionTests
• Thyroglobulin
– Protein made and secreted by follicular cells
– Presence indicates thyroid tissue is present
– Used as a tumor marker for thyroid cancer
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
• 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
Common Thyroid Function Tests
•
•
•
•
•
TSH
Total T3 and total T4
Free T3 and Free T4
T3 Uptake (for binding proteins)
Serological testing for antibodies to thyroid
disease
Thyroid Testing Methods
• Competitive immunoassay
– RIA
– ELISA
– FIA
• Noncompetitive immunoassay
Thyroid Antibodies
• Thyroglobulin antibodies
– Anti-thyroglobulin antibodies are mostly IgG and
do not fix complement
– Found in
•
•
•
•
Hashimoto’s thyroiditis
Grave’s disease
Subacute thyroiditis
Thyroid cancer
References
• Bishop, M., Fody, E., & Schoeff, l. (2010).
Clinical Chemistry: Techniques, principles,
Correlations. Baltimore: Wolters Kluwer
Lippincott Williams & Wilkins.
• Sunheimer, R., & Graves, L. (2010). Clinical
Laboratory Chemistry. Upper Saddle River:
Pearson .