Yamashita et al evaluated transient post-operative tetany that occurred in women with Graves's Disease who underwent subtotal thyroidectomy. They found that the presence of both vitamin D deficiency and elevated alkaline phosphatase levels posed the greatest risk for transient post-operative tetany. The authors are from the Noguchi Thyroid Clinic in Beppu, Oita, Japan.
Mechanisms proposed to explain tetany following thyroid resection:
(1) hypoparathyroidism caused by either surgical manipulation or removal of the parathyroid glands
(2) abnormal release of calcitonin during surgical manipulation
(3) thyrotoxic osteodystrophy
(4) preoperative secondary hyperparathyroidism caused by calcium and vitamin D deficiencies
The hypoparathyroidism and tetany may be permanent if the parathyroid glands are removed or damaged during surgery. The risk for this is minimal if (1) the surgeon is well-trained, (2) the parathyroid glands are meticulously handled to minimize devascularization, and (3) excised or devascularized parathyroid glands are immediately reimplanted.
Yamashita et al (1997) had previously demonstrated that female gender was the most important risk factor for transient post-operative tetany following subtotal thyroidectomy for Graves's Disease. They found that tetany was due to a combination of secondary hyperparathyroidism (mechanism 4) and transient hypoparathyroidism due to manipulation of the glands (mechanism 1).
Parameters found associated with transient post-operative tetany in women:
(1) serum alkaline phosphatase
(2) 25-hydroxy Vitamin D
|
Assay
|
Lower Limit of Normal Reference Range
|
Higher Limit of Normal Reference Range
|
|
alkaline phosphatase
|
43 U/L
|
155 U/L
|
|
25-Hydroxy Vitamin D [25 (OH) D]
|
25 nmol/L
|
137 nmol/L
|
|
25-Hydroxy Vitamin D
|
Serum Alkaline Phosphatase
|
Incidence of Post-Operative Tetany
|
|
<= 25 nmol/L
|
> 155 U/L
|
19.1%
|
|
<= 25 nmol/L
|
<= 155 U/L
|
11.8%
|
|
> 25 nmol/L
|
> 155 U/L
|
6.7%
|
|
> 25 nmol/L
|
<= 155 U/L
|
0%
|
after Figure 1, page 468
The impact of postoperative tetany in these patients should be reduced by
(1) preoperative correction of calcium and vitamin D deficiencies
(2) meticulous handling of the parathyroid glands during surgery
(3) careful observation of patients with elevated serum alkaline phosphatase
The authors noted that the onset of post-operative tetany was usually within 24 hours after surgery. This means that patients without tetany can be discharged 24 hours after the operation, although they should be instructed to call if symptoms of hypocalcemia develop.
