Stress (cont.)
During stress, the priorities of the secretions of the hypothalamo-pituitary peripheral-endocrine axes
are shifted in favor of the hypothalamo-pituitary-adrenocortical (HPA) axis.
During stress:
HPA
GHRHGHGnRHFSHLH
Hypothalmo-Pituitary-Thyroid System
Figure 13-1: Diagrammatic representation of the thyroid gland
Figure 13-2
Table 13.3Major Actions of Thyroid
Hormones• Calorigenesis
• Metabolism• Brain maturation
• Behavior• Growth & development
Figure 13-3
CNS
↓HYPOTHALAMUS
TRH
↓PITUITARY
↓TSH
↓THYROID GLAND↓ ↓ ↓T3 T4 rT3
TARGET CELLS
↓T4 T3
↓INTRACELLULAR (NUCLEAR) BINDING
↓METABOLI C RESPONSE
CLEARANCE
FREE &BOUND
(-)
(-)
3, 5, 3’, 5’ Tetraiodothyronine (thyroxine, T4)
3, 5, 3’ Triiodothyroine (T3)
Table 13-2: Some MORPHOLOGIC Changes in the Thyroid Gland with Aging
FOLLICLES:- Are distended- Change in color
- Epithelium flattened w/reduced secretion
Fewer mitoses
Increased connective tissue;
Fibrosis
Atherosclerotic changes
Table 13-2 (con’t.): Some SECRETORY Changes
in the Thyroid Gland with Aging
Simultaneously decreased secretion and metabolic
clearance of T4 with resulting essentially normal levels
Failure of up-regulation of
T3 nuclear receptors
peripheral conversion of T4 to T3
TSH levels in 10% of the elderly, associated
in antithyroid antibodies, present even in the absence of manifestations of hypothyroidism
circulating T3 levels but generally within
the normal (lower) range
Table 13-1: Some Critical Aspects of Thyroid Hormone Regulation
1. Major source of circulating T3 from peripheral deiodination of T4 (NOT from thyroid gland secretion)
2. The negative feedback at the pituitary anterior lobe is mainly through T4 (taken from circulation & converted into T3)
3. The peripheral deiodination of T4 depends on the physiological state of the organism. It allows an autonomy of response of the tissues to the hormones.
4. Deiodination can convert T4 (a less biologically active hormone) to T3 (a more active hormone). This conversion depends on the activity of the various deiodinating enzymes.
Figure 13-4
Basal Metabolic Rate
Basal Metabolic Rate
from birth to 70 years old
Table 13-5 In the Elderly, Thermoregulatory Insufficiency
Results from:
heat production,
body mass,
muscle activity,
shivering,
sweating response,
vasomotor responses,
temperature perception.
Table 13-6 Autoimmune Diseases of the Thyroid Gland
Characteristics Graves’ Disease Hashimoto’s Thyroiditis
Thyroid Status Hyperthyroid Hypothyroid
TSH Generally undetectable
Normal to elevated
T4, T3 (serum) Above normal Below normal
Antibodies(ABs) Stimulatory ABs compete with TSH at receptor sitesLoss of TSH control over thyroid function
Some ABs block TSH actions
Autoantibodies against thyroglobulin, T3, T4, thyroid destroy thyroid microsomal and nuclear components
Generally present
Generally present
Lymphocytic Invasion
Limited Marked
Female:Male Ratio
As high as 10:1 As high as 10:1