section 33.2

Thyroid Hormone Synthesis

773

C old

S tr e s s

X

&

Brain

O ther

stimuli

A drenergic neural netw ork

for growth differentiation and

m ental d evelop m en t

F IG U R E 3 3 -4

Schematic representation of the major steps in the regulation of thyroid hormone secretions and metabolism at five

levels, namely, brain, hypothalamus, pituitary thyrotropes, thyroid, and peripheral tissues.

Release of TSH from the anterior pituitary is stimulated

by hypothalamic TRH and inhibited by thyroid hormone

(Chapter 31). These opposing signals to the anterior pi-

tuitary determine the magnitude of TSH secretion. Many

other factors influence secretion of TSH and thyroid hor-

mone (Figure 33-4). For example, starvation reduces con-

version of T

4

to T

3

in peripheral tissues but not in the

anterior pituitary; consequently, TSH secretion is not sup-

pressed but circulating T

3

levels fall. Stress, somatostatin,

estrogens, and cold exposure (in infants) also affect TSH

release, but except for cold stress in infants, they are

not regarded as important regulators of TSH and thyroid

hormone secretion.

Iodine

The thyroid gland is capable of adjusting its synthetic

activity to the supply of iodine. When the iodine supply

is low, the thyroid makes maximal use of iodide; when

the iodide supply is abundant, the thyroid defends itself

against hormone overproduction by reducing iodide up-

take. The thyroid gland autoregulates its iodine supply

by an internal feedback mechanism that controls the in-

traglandular handling of iodide and the thyroid response

to TSH. The exact nature of this feedback is not known,

but the amount of the organified iodide pool is believed

to have an inhibitory effect on the rate of iodide uptake,

intraglandular T

4

/T3, the completeness of thyroglobulin

hydrolysis, and the responsiveness to TSH.

This autoregulatory mechanism is not prominent when

iodine intake is within the normal range but becomes phys-

iologically significant when there is a moderate deficiency

or excess of iodine. On an ordinary diet in which the

iodine intake ranges between 300

fig

and 1000 /zg/d, T

4

production increases linearly with increased consumption