Thyriod Function Flashcards
Thyroid General Info
Held around trachea loosely by connective tissue
3-5 cm long
15-25 g
Thyroid Gland Microscopic Info
2 types of cells
- Follicular cells
- Parafollicular cells
Follicular Cells
AKA fundamental unity of the thyroid gland
Produces thyroglobulin (glycoprotein) secreted into the interior of follicle
Site of production of thyroid hormones
Parafollicular Cells
AKA C-Cells
Secrete calcitonin
Calcitonin
Protein involved in metabolism of Calcium and Phosphorus
Physiological Effects of Thyroid Hormones
Calorigenic
Support growth, development, sexual & CNS maturation
Increase Protein synthesis
Increase heart rate and GI tract motility
Increase Sensitivity of cell receptors to catecholamines
Organs Unaffected by Thyroid Hormones
Brain
Retina
Lungs
Spleen
Testes
Calorigenic
Increase O2 consumption and metabolic rate
Source of Iodide
Dietary
Trapped from the circulation
Iodide Oxidized
Inside Follicle
Attaches to tyrosine residue of thyroglobulin
MIT
Monoiodotyrosine
DIT
Diiodotyrosine
Iodination of Thyroid Hormones
Synthesis is under action of thyroid peroxidase (TYR) and thyroid stimulating hormone (TSH)
T3
Made when MIT and DIT are combined
T4
Made when 2 DITs are combined
Colloid
Located in Follicular
Stores thyroid hormones
Negative Feedback for T3 & T4
Stimulation of the pituitary
Peptide bonds between tyrosine and thryoglobulin are cleaved by thyroglobulin proteases
Thyroxine (T4)
Major Hormone (10X T3)
Deiodinated to T3 in peripheral tissue (liver)
Less biologically active than T3
Triiodothyronine (T3)
Produced in less concentration than T4
More biologically active (3-5X T4)
Reverse T3 (rT3)
Formed from deiodination of T4
Not biologically active
T4
T3
rT3
Transport of Thyroid Hormones
Circulate bound to proteins
Free form is physiologically active form
TBG
Thyroid Binding globulins
Albumin
Catch all carrier protein
Free T4
fT4
0.03%
Free T3
fT3
0.3%
Bound T4
99.97%
Bound T3
99.7%
Testing for Thyroid
Competitive binding immunoassay
Stripping agent is added to remove TBG and other binding proteins
Thyroid Antibodies
Previously thyroid microsomal antibodies
Hashimoto’s Thyroiditis - 80-99%
Grave’s Disease - 45-80%
Antithyroglobulin Antibodies
Hashimoto’s thyroiditis - 6-60%
Grave’s Disease 70-100%
Anti-TSH receptor
Hashimoto’s thyroiditis - 6-60%
Grave’s Disease - 70-100%
TRH Stimulation Test
Inject patient with synthetic TRH after collection of baseline TSH
Collect TSH samples at 15, 30, and 60 minutes
Flat response = inability to produce TSH
Reverse T3 Info
Liver converts T4 to T3, which also produces a certain % of RT3
When body is under stress, it can prevent tissues that depend on T3 from producing T3 by producing RT3
Associated with non-thyroidal illness (NTI)
Testing remains controversial
Radioiodine Uptake Test
Patient given radioisotope of iodine
Gives a picture of areas of gland that are utilizing iodine
Ultrasound Evaluation
Detection of small nodules
Abnormal thyroid anatomy
Hypothyroidism
Deficiency in thyroid hormone activity
Can be structural or functional
Hypothyroidism Occurence
As high as 15% with female preference
Primary Hypothyroidism
Deficiency in thyroid hormone production
Secondary Hypothyroidism
Pituitary or hypothalamic failure
- Sometimes called tertiary
Clinical Signs and Symptoms of Hypothyroidism
Slowing of metabolism
Fatigue
Cold intolerance
Weight gain
Constipation
Dry Skin
Mental Slowing
Myxedema (skin on face and feet thickens)
Tongue and vocal cords may thicken
Face may become puffy
Etiology of Hypothyroidism
Acquired secondary to thyroid surgery or treatment
Inflammation = Thyroiditis
Acute Thyroiditis
Infection with Gram Positive Cocci
(S. pyogenes, S. pneumoniae, S. aureus)
Subacute Thyroiditis
Usually viral in origin
Chronic Thyroiditis
Hashimoto’s
Hashimoto’s Thyroiditis
Most common cause in US
Women > Men
Genetic predisposition
Present with goiter
Neoplasms Hypothyroidsm
Rare in US
Papillary carcinmona ( young patients & women)
- Forms a firm, solitary nodule
Follicular carcinoma
Middle age and can metastasize
Medullary carcinoma
Parafollicular cells
Primary Congenital Hypothyroidism
Creatinism
Creatinism
Deficiency in thyroid tissue
Inborn defects and pituitary or hyopthalamic disorders
Untreated congenital hypothyroidism can cause developmental defects
Creatinism Symptoms
Respiratory distress
Skeletal malformations
Hypothermia
Physiologic jaundice beyond 3 days
Edema
Hoarse cry
Thick tongue
Mental retardation
Creatinism Screening
State Health Departments as a newborn
Thyroid Testing for Primary Hypothyroidism
TSH - increased
fT4 - decreased
T3 - decreased
TSH is the single most sensitive test for hypothyroidism
Hyperthyroidism
Increase in circulating hormone both free and total = Thyrotoxicosis
Weight loss
Nervousness
Sweating
Heart palpitation
Heat intolerance
Muscle weakness
Goiter - Enlarged thyroid gland
Exophthalmos (eyelid retraction)
Hyperthyroidism - Thyroid Storm
Emergency Situation
Associated with infection, childbirth, diabetic ketoacidosis
Tachycardia, heart failure, high fever, nausea, and vomiting
Neurologic and psychiatric disorders, coma
Untreated: 70-80% cases fatal
Hyperthyroidism Excess Stimulators
Grave’s Disease - Toxic Diffuse Goiter
Grave’s Disease Info
10X more frequent in women
30-50 years of age
Autoimmune
Follicles stimulated to produce excess T4
Not under normal negative feedback regulation
Symptoms of Grave’s Disease
Hyperthyroidism
Goiter
Exopthalmos
Hyperthyroidism - Other Causes
Toxic nodular goiter (nodules secrete hormone)
Iatrogenic and factitious causes
Primary Hyperthyroidism Testing & Treatment
TSH - decreased
fT4 - increased
T3 - increased
Ablation
Medical
Radiation
Euthyroid Syndromes
Refers to a condition in which thyroid function tests are abnormal in seriously ill patient
Patient does NOT have Primary or Secondary thyroid disease
Metabolism of hormones at peripheral tissues is altered or binding protein concentration is altered (usually decreased)
Non-Thyroidal Illness (NTI) or Euthyroid Sick Patients
Low TBG
ICU patients
Drug therapy = Decreased TBG
Test Summary for NTI
Total T4 = Decreased
- Amount of binding protein is decreased
- Artifactual result of test method
Free T4 = Decreased
TSH = Decreased
T4 = Decreased
rT3 = Increased
Euthroid Estrogen Therapy
High TBG
Increased TBG from estrogen
Euthroid Estrogen Causes
Birth Control Pills
Pregnancy
Estrogen Therapy
Test Summary for Euthroid Estrogen
Total T4 = Increased
fT3 = Normal
fT4 = Normal
Summary of Primary Hypothyroidism
Total T4 = Decreased
Free T4 = Decreased
TSH = Increased
Summary of Secondary Hypothyroidism
Total T4 = Decreased
Free T4 = Decreased
TSH = Decreased
Summary of Euthyroid Sick
ICU/Drugs
Dependent on degree of illness
Total T4 = Decreased
Free T4 = Decreased to Normal
TSH = Decreased to Normal
Summary of Primary Hyperthyroidism
Total T4 = Increased
Free T4 = Increased
TSH = Decreased
Summary of Secondary Hyperthyroidism
Total T4 = Increased
Free T4 = Increased
TSH = Increased
Summary of Euthyroid Estrogens
Estrogen Therapy & Pregnancy
Total T4 = Increased
Free T4 = Normal
TSH = Normal
