A major complication of thyroidectomy is subsequent hypocalcaemia.
The cause is not always clear; although damage to parathyroid tissue due
to poor surgical technique or misfortune may occur it is known that hyperthyroid
cats often have high parathyroid hormone (PTH) levels and a drop
in PTH may be expected in some individuals just from removal of thyroid
tissue. All cats should be closely observed for a week post thyroidectomy
and have serum/ plasma total calcium levels monitored for three days. Good
surgical practice, staged unilateral procedures at a three-week interval
and burial of any excised parathyroid tissue in adjacent muscle are all
likely to prevent permanent hypoparathyroidism.
The clinical signs of hypoparathyroidism occur in one to
seven days after surgery; they result from hypocalcaemia and include: decreased
appetite, facial rubbing, muscle twitching, panting, weakness, nervousness,
stiff gait, seizures and tetany. Low blood calcium without clinical signs
does not require treatment but may indicate the requirement for enhanced
observation, monitoring and prolonged hospitalization.
Therapy for hypocalcaemia due to hypoparathyroidism follows
two strands: calcium to initially stabilise blood levels and then to ensure
ample calcium intake and vitamin D, or an analogue, to control blood calcium
level more chronically. Calcium must be given according to need: if there
are life-threatening signs of hypocalcaemia then the calcium may be given
slowly i/v (calcium gluconate 10% 0.5-1.5 ml/kg) over 10 minutes,
with concurrent ECG (or at least stethoscope) monitoring; infusion
must stop if there is a bradycardia. Subsequently, s/c calcium gluconate
is given (1-2 ml 10% calcium gluconate diluted 1:1 with saline s/c tid);
and then oral calcium carbonate (0.25 to 1 gm/day oral divided dose).
The most active form of vitamin D is calcitriol (1,25-dihydroxycholecalciferol)
[Rocaltrol, Roche], the dose is 0.01-0.03 ug/kg po sid; however, it is manufactured
in inconvenient 0.25 ug capsules (containing a liquid). Another active
form is alfacalcidol [One-Alpha, Leo], the dose is 0.05 ug/kg po sid and
it is available as a liquid. Alternatively, dihydrotachysterol [AT10, Sanofi
Winthrop] at 0.02-0.03 mg/kg sid for 2-3 days, then 0.01-0.02 mg/kg/day
with adjustments based on the blood total calcium. Calcitriol and alfacalcidol
take 1-4 days to reach peak activity from commencement of therapy, dihydrotachysterol
takes 1-7 days, and during this time parenteral calcium therapy is necessary.
Once stable, the blood calcium is measured every week and the dose of the
vitamin D supplement is adjusted by about 20%, as necessary, to maintain
the calcium in the low-normal range. Calcitriol and alfacalcidol cease to
affect calcium levels a day after withdrawal; however, dihydrotachysterol
takes 1-3 weeks to have no activity.
The owners must be informed about the clinical signs of
hypercalcaemia (anorexia, lethargy, vomiting). If iatrogenic hypercalcemia
develops the medications are discontinued until calcium falls to the low-normal
range, i/v saline and frusemide may be administered. Vitamin D and calcium
medications are only reinstated if hypocalcaemia reoccurs and then at a
20% reduced dose. The medications can usually be gradually withdrawn, starting
after 3-4 weeks, and with a monitored reduction in dose of 25-50% each week.
Rarely does medication have to be continued indefinitely. |