- Table showing common interpretations for TBG and T3 resin uptake*
|
Primary Condition
|
Test result
|
| Hyperthyroidism
|
High T4, High T3 resin uptake, Low TBG
|
| Hypothyroidism
|
Low T4, Low T3 resin uptake, High TBG
|
| High TBG
|
High T4, Low T3 resin uptake
|
| Low TBG
|
Low T4, High T3 resin uptake
|
*T3 resin uptake is used to indirectly measure TBG levels.
- Somatostatin: It is secreted by the delta cells of the pancreas. It inhibits the secretion of both insulin and glucagon.
- Renin - angiotensin - aldosterone system (RAA): Renin is an enzyme that cleaves angiotensinogen to angiotensin I, which in turn is cleaved by angiotensin converting enzyme or ACE to angiotensin II. ACE is present in the vascular endothelium of the lungs and brain, heart etc. Further breakdown produces angiotensins III and IV. Angiotensin II, III and IV stimulate aldosterone secretion and vasoconstriction, while angiotensin II is more potent for vasoconstriction.
Angiotensin II functions
- Angiotensin II acts on AT1 receptors on blood vessels
- Vasoconstriction and increased peripheral resistance and blood pressure
- Stimulates Na+ reabsorption by the kidney
- Stimulates the adrenal cortex to release aldosterone
- Stimulates the release of ADH from the posterior pituitary
- Stimulates thirst
- Facilitates the release of norepinephrine from the sympathetic nerve endings
Release of renin is stimulated by beta 1 stimulation, renal artery hypotension due to any cause and decreased Na+ delivery to the distal renal tubules. Renin is produced and secreted by juxtaglomerular or JG cells of the afferent arteriole in the renal glomerulus. Specialized cells of the distal tubules called macula densa lie adjacent to the JG cells. The macula densa senses the concentration of Na+ and Cl- reaching the distal renal tubule. When NaCl is elevated in the tubular fluid, renin release is inhibited. In contrast, a reduction in tubular NaCl stimulates renin release by the JG cells. Similarly, a reduction in afferent arteriole pressure causes the release of renin from the JG cells, whereas increased pressure inhibits renin release. PG E2 and PGI2 stimulate renin release in response to reduced NaCl.
