Functions Of T3 AND T4 include the following

A. Increase the basal rate of oxygen consumption and heat production

B. Increase cardiac output

C. Increase gluconeogenesis and glycogen degradation

D. Stimulate cartilage growth

E. Stimulate endochondral ossification and linear growth of bone

F. Play a crucial role in central nervous system (CNS) development (a deficiency of T, and T4 results in permanent brain damage)

Vesicles

Lysosomes

Golgi complex

Deiodinase

'TSH receptor

Na+I" cotransporter jJH2 CH2CHCOOH

CH2CHCOOH

Parafollicular cells

Calcitonin

Capillary

T3 T4

Thyroid peroxidase

Figure 21-1. Diagram of triiodothyronine (T}) and thyroxine (T4) synthesis and secretion by follicular cells. The secretion of calcitonin by parafollicular cells also is shown. DIT = diiodotyrosine; MIT = inonoiodotyro-sine; TBG = thyroid-binding globulin; TG = thyroglobulin; TSI I = thyroid-stimulating hormone.

IV. PARAFOLLICULAR CELLS secrete calcitonin, which acts directly on osteoclasts to decrease bone resorption, thereby lowering blood calcium levels.

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