section 33.2
Thyroid Hormone Synthesis
C old
S tr e s s
O ther
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
to T
in peripheral tissues but not in the
anterior pituitary; consequently, TSH secretion is not sup-
pressed but circulating T
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.
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
/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
and 1000 /zg/d, T
production increases linearly with increased consumption
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