Thyroid Gland

A. Location - lower aspect of larynx and upper trachea

- two lateral lobe connected by an isthmus on anterior

surface of superior trachea

B. Histology & Physiology

1. Thyroid Follicles - functional units

a. follicle cells secrete Thyroxine (Thyroid Hormone)

Thyroxine is a combination of Tyrosine & Iodine to make

two compounds: Tetraiodothyronine – T₄

Triiodothyronine – T₃

Thyroxine is transported in the blood as Thyroid-binding

Globulin (TBG)

2. Test for Thyroid Activity –

- thyroxine is transported in combination with plasma proteins

(TBG) – PBI test ( protein Bound Iodine) Serves to quantitify

the levels of thyroxine in the blood

3. Thyroxine Action –

a. Calorigenic Effects – increases Catabolism and produces extra body heat

1 mg of thyroxine = an increased heat production of 1000 calories

b. Growth & Development – Works with STH to produce growth and

development of tissues and organs

- Nervous Tissue particularly effected

- hyposecretion results in Mental Retardation

4. Metabolic Effects –

a. diuretic

b. protein and carbohydrate catabolism (stimulates cellular uptake)

c. increases activity of nervous system

d. increases heart rate

e. causes muscular weakness

5. Parafolliclar Cells – secrete Thyrocalcitionin (TCT)

a. decreases excretion of Calcium & Phosphate ions

b. increases Osteoblast activity –[Ca⁺² & PO^-3 deposition in bone

6. Disorders of the Thyroid

a. Hyperthyroidism (Grave’s disease)

- elevated PBI

- increases nervousness and irritability

- elevated BMR

- exophthalmos – results in edema behind the eyes

b. Hypothyroidism

Cretinism – occurs if the hyposecretion is during fetal or

early developmental life.

- results in reduced metabolism

- results in reduced growth

- results in mental retardation

Myxedema – occurs if the hyposecretion is during adult life

- results in reduced metabolism

- results in mental & physical activity

- results in increased blood pressure

- results in accumulation of subcutaneous fluids

Category: Physiology Notes