Chem: Section 11.3- Thyroid Endocrinology

What are some of the many functions of the thyroid hormones T4 and T3?

- Growth and maturation of bones and other tissues of the skeletal system, including nerves in the CNS

- Regulation of growth hormones

- Sexual maturation

- Body temperature regulation and generation of heat

- Metabolic functions

- Regulation of cardiac output as determined by heart rate and respiration rate

- Mood and behavior

- Fetal development, esp. neurologic development

What is TSH composed of?

peptide hormone composed of two polypeptide alpha and beta chains

How is TSH transported through the body?

is a protein hormone, so transported free (not bound to a carrier)

Where does TSH bind?

is a protein hormone, so binds to a receptor on the *surface* of the cell (activates adenylate cyclase, cAMP)

What is the other name for TSH?

thyrotropin

Where are thyroid hormones synthesized? What are they made from?

synthesized in the thyroid gland

derived from tyrosine and iodine

How much T4 and T3 does the thyroid gland produce?

mostly T4, some T3

thyroid hormone that best reflects granular secretion

T4

most common carrier protein for thyroid hormones

thyroid-binding globulin (TBG)

Which form of the thyroid hormones is biologically active?

the unbound (free) portion

(most of it is bound to TBG, so this is a small portion of total thyroid hormone)

What are some factors that *increase* TBG levels?

pregnancy, liver disease, estrogen therapy, medications

What are some factors that *decrease* TBG levels?

malnutrition (not ingesting protein), renal or liver disease (nephrotic, cirrhosis), major illness

What are some S/S associated with *high* levels of thyroid hormones?

- Increased body temperature

- Increased basal metabolic rate

- Increased cardiac output/cardiac rate

- Increased respiratory rate

- Influence mood and behavior

What are some S/S associated with *low* levels of thyroid hormones?

- Decreased body temperature

- Decreased basal metabolic rate

- Decreased cardiac output/cardiac rate

- Decreased respiratory rate

- Influence mood and behavior

If a patient exhibits S/S of a thyroid disorder, the first test that should be ordered is what? What are these results compared to?

TSH

compare the TSH to the age and gender adjusted reference ranges, if they are abnormal thyroid disease is likely present

If the TSH result does not correlate with the S/S of the patient, what test is ordered next?

free T4

In adults, a TSH is ordered first, followed by a T4 if necessary. Are neonates tested in the same manner?

No. In neonates, a T4 will be tested first, followed by a TSH

In what case is a T3 ordered?

suspected cases of subclinical hyperthyroidism (would show a normal TSH and T4)

disorder that will show normal to low thyroid hormone levels, and is usually from a secondary cause

euthyroid sick syndrome

antibodies directed against the thyroglobulin protein located within the thyroid colloid, which are the hollow follicular sacs of the thyroid gland

thyroglobulin antibodies

antibodies directed against a protein component of thyroid cell microsomes

microsomal antibodies

Which thyroid antibody test is preferred? Thyroglobulin or microsomal?

microsomal (although many physicians will order both)

High titers of thyroid antibodies are associated with what two conditions?

Hashimoto's thyroiditis and Grave's disease

autoimmune primary hyperthyroidism where antibodies bind to TSH receptor and stimulate the thyroid gland to over-secrete

Grave's Disease

What classic S/S are associated with Grave's disease? (2)

toxic diffuse goiter and exophthalmos (bulging eyes)

autoimmune primary hypothyroidism characterized by persistent inflammation that causes chronic thyroid damage, leading to a drop in thyroid gland secretion

Hashimoto's Disease

What type of thyroid condition presents with a goiter?

Primarily hyperthyroidism, but can sometimes see it in hypo due to the inflammation

What are some risk factors that may result from hypothyroidism during pregnancy?

- Diminished intellectual capacity and developmental delay in children

- Miscarriage

- Preterm delivery

- Preeclampsia

- Placental abruption in mothers

- Stillbirth

What are some common S/S of hyperthryoidism?

heat intolerance, tachycardia, weight loss or inability to gain weight, weakness, excessive emotional reactions and mood swings, tremors, goiter, optic changes including exophthalmos (bulging eyes), atrial fibrillation

What are some common S/S of hypothryoidism?

bradycardia, hoarseness, cold sensitivity, dry skin and hair, muscle weakness, weight gain/box-shape, personality change, memory impairment, myxedema (swelling of face), decreased libido, cretinism

condition that results from when the patient is hypothyroid at a young age and is never treated

cretinism

What are the following secreted by?

TRH

TSH

T4/T3

TRH: hypothalamus

TSH: anterior pituitary

T4/T3: thyroid

What is the most common methodology for measuring the thyroid hormones?

immunoassay

What type of blocking reagent is required for total T4 measurement?

blocking agent needed to displace the thyroxine from the binding proteins, since 99% of the hormone is bound

What type of blocking reagent is required for free T4 measurement?

must separate trace amounts of free T4 from the protein-bound T4

What type of blocking reagent is required for total T3 measurement?

the immunoassay uses an anti-serum that does not react with T4 and also uses a blocking agent to release bound T3

What are the S/S of Hashimoto's Disease?

inflammation

same as primary hypo:

bradycardia, hoarseness, cold sensitivity, dry skin and hair, muscle weakness, weight gain/box-shape, personality change, memory impairment, myxedema (swelling of face), decreased libido, cretinism