Page 376 - First Aid for the USMLE Step 1 2020, Thirtieth edition [MedicalBooksVN.com]_Neat
P. 376
332 SEcTion iii EndocrinE ` endocrine—Physiology EndocrinE ` endocrine—Physiology
Parathyroid hormone
soUrce Chief cells of parathyroid
2+
3–
2+
FUnction free Ca in the blood (1° function) PTH serum Ca , serum PO 4 , urine
3–
3–
2+
Ca and PO 4 absorption in GI system PO 4 , urine cAMP
2+
3–
Ca and PO 4 from bone resorption RANK-L (receptor activator of NF-κB ligand)
2+
Ca reabsorption from DCT secreted by osteoblasts and osteocytes; binds
3–
PO 4 reabsorption in PCT RANK (receptor) on osteoclasts and their
2+
1,25-(OH) 2 D 3 (calcitriol) production by precursors to stimulate osteoclasts and Ca
activating 1α-hydroxylase in PCT bone resorption (intermittent PTH release
Tri to make D 3 in the PCT can also stimulate bone formation)
PTH = Phosphate-Trashing Hormone
PTH-related peptide (PTHrP) functions
like PTH and is commonly increased in
malignancies (eg, squamous cell carcinoma of
the lung, renal cell carcinoma)
2+
regUlAtion serum Ca PTH secretion
3−
serum PO 4 PTH secretion
2+
serum Mg PTH secretion
2+
serum Mg PTH secretion
2+
Common causes of Mg include diarrhea,
aminoglycosides, diuretics, alcohol abuse
PTH activity
2+
3–
↓ ionized Ca , ↑ PO , or 1,25-(OH) D
4 2 3
↑
Four
para- Vitamin D activity
thyroid
Feedback glands
inhibition 25-OH D 3
of PTH
synthesis
PTH released PO 3–
into circulation ↑ 4
1α-hydroxylase
↑ 1,25-(OH) D
2 3
1,25-(OH) D 3
2
Renal tubular cells
Bone Intestines
2+
↑ 1,25-(OH) D synthesis ↑ Ca and ↑ PO 4 3– ↑ absorption of
2
3
3–
2+
Reabsorption: ↑ Ca , PO 4 3– released from bone Ca and PO
2+
4
↑
Urine Ca , ↑ PO 3–
2+
4
↑
↑ Ca and ↑ PO 3–
2+
4
2+
↑ Ca and PO 3–
4
↑
FAS1_2019_08-Endocrine.indd 332 11/7/19 4:30 PM
