1. MLAB 2401: Clinical Chemistry Keri Brophy-Martinez
Thyroid Gland

2. Thyroid Anatomy Largest endocrine gland
Located in the lower part of the neck, wrapped around trachea

3. Thyroid Cells Composed of two cell types Follicular
Make and secrete thyroxine (T4) and triiodothyronine (T3)
Parafollicular/ “C” cells
Secrete calcitonin

4. 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

5. Synthesis of Thyroid Hormones

6. Thyroid Hormones Triiodothyronine (T3) Active form of the hormone
More potent than T4
Thyroxine (T4)
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

7. 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

8. 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

9. Actions of Thyroid Hormone
Regulation of body metabolism
Body temperature
Increases heat production
Energy use
Calorie burning
Oxygen consumption
Brain maturation
Tissue growth

10. Disorders of the Thyroid Gland

11. Terms Euthyroid Goiter Normal thyroid gland function
Condition where the thyroid gland grows larger than normal

12. Hypothyroidism Symptoms Lab features
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

13. 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

14. Thyrotoxicosis Causes of: Excessive thyroid hormone ingestion
Leakage of stored thyroid hormone from thyroid follicles
Excessive production of thyroid hormone (hyperthyroidism)

15. Thyrotoxicosis Signs Symptoms Tachycardia Tremor
Warm, moist, flushed skin
Goiter
Muscle wasting and weakness
Anxiety, nervousness
Palpitations
Weight loss
Heat intolerance
Prominence of eyes

16. Graves’ Disease Autoimmune Disease Familial Women more affected
Antibodies activate TSH receptor
Familial
Women more affected
Lab Features
High free T4 and/or T3 level
TSH low-undetectable

17. Thyroiditis Condition where there is inflammation of the thyroid gland
Categories include acute, subacute, chronic

18. 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

19. Assessment of the Thyroid

20. 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

21. 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

22. 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

23. Thyroid Function Tests
Free T3/ Triiodothyronine
Measure free/unbound T3
Shows the level of hormone available for uptake & use

24. Thyroid FunctionTests
Thyroglobulin
Protein made and secreted by follicular cells
Presence indicates thyroid tissue is present
Used as a tumor marker for thyroid cancer

25. 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

26. 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

28. Thyroid Testing Methods
Competitive immunoassay
RIA
ELISA
FIA
Noncompetitive immunoassay

29. 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

30. 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 .