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

2. 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

3. 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 Iodide
seafood, dairy products, vitamins and iodine-enriched breads

4. Synthesis of Thyroid Hormones

5. 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

6. 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

7. 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

8. Actions of Thyroid Hormone
Tissue growth
Brain maturation
Increased heat production
Increased oxygen comsumption

9. Disorders of the Thyroid Gland

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

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

12. Hypothyroidism Types Dysfunction Condition Primary Thyroid gland
Hashimoto’s thyroiditis
Treatment for toxic goiter
Excessive iodine intake
Subacute thyroiditis
Secondary
Pituitary gland
Hypopituitarism
Tertiary
Hypothalamus
Hypothalamic

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

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

15. 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 undetectable

16. Hyperthyroidism

17. 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

18. 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.

19. Thyroid Nodules Common 5–9% prove to be thyroid cancer.
Clinically apparent nodules are present in 6.4% of adult women & 1.5% of adult men.
Thyroid ultrasound finds unsuspected nodules in 20–45% of women & 17–25% of men.
5–9% prove to be thyroid cancer.
Fine-needle aspiration, with cytologic examination of aspirate, is used to determine need for surgical removal.

20. Assessment of the Thyroid

21. 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

22. 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

23. 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

25. Thyroid Testing Methods
Competitive binding techniques
RIA
ELISA
FIA

26. Thyroid Antibodies Thyroglobulin antibodies
Anti-thyroglobulin antibodies are mostly IgG and do not fix complement
Titerable using an indirect hemagglutination assay
Found in
Hashimoto’s thyroiditis
Grave’s disease
Subacute thyroiditis
Thyroid cancer

27. Thyroid Antibodies Microsomal antibodies
Complement fixing IgG directed against a microsomal antigen within follicular cells
Found in
Hashimoto’s disease
Thyrotoxicosis
Primary myxedema