Dihydrotachysterol (Vitamin D)

Dihydrotachysterol is a synthetic vitamin D analogue used to increase calcium availability in patients with persistent hypocalcemia. Its principal veterinary application is management of hypoparathyroidism in dogs and cats, including hypocalcemia after thyroid surgery. Its activation pathway allows a calcemic effect even when parathyroid-hormone-dependent vitamin D activation is inadequate.

Group

Vitamin D analogue

Mechanism of Action

Dihydrotachysterol undergoes hepatic hydroxylation to an active metabolite that exerts vitamin-D-like effects without requiring the usual renal 1-alpha-hydroxylation step. It increases intestinal calcium absorption and can increase calcium mobilization from bone. These actions raise extracellular calcium availability and compensate for the impaired calcium regulation associated with deficient parathyroid hormone.

Dose Rates

Dog

Use / stage	Dose	Route	Schedule	Instructions
Hypocalcemia	0.01 mg/kg	PO	Every 24 h
Acute hypocalcemia — Initial	0.02 mg/kg	PO
Acute hypocalcemia — Maintenance	0.01–0.02 mg/kg	PO	Every 24–48 h	Adjust to serum calcium. Higher 0.1–0.3 mg/kg doses are mentioned without a defined schedule and are not default escalation doses.
Hypoparathyroidism induction taper	Reference only	PO		Start 0.03 mg/kg daily until a response, then 0.02 mg/kg daily for 2 days, followed by 0.01 mg/kg daily. Maintain inpatient monitoring until calcium is stable at 8–9.5 mg/dL; check weekly initially and every 2–3 months during stable long-term treatment.

Cat

Use / stage	Dose	Route	Schedule	Instructions
Hypocalcemia	0.01 mg/kg	PO	Every 24 h
Acute hypocalcemia — Initial	0.02 mg/kg	PO
Acute hypocalcemia — Maintenance	0.01–0.02 mg/kg	PO	Every 24–48 h	Adjust to serum calcium. Higher 0.1–0.3 mg/kg doses are mentioned without a defined schedule and are not default escalation doses.
Hypoparathyroidism induction taper	Reference only	PO		Start 0.03 mg/kg daily until a response, then 0.02 mg/kg daily for 2 days, followed by 0.01 mg/kg daily. Maintain inpatient monitoring until calcium is stable at 8–9.5 mg/dL; check weekly initially and every 2–3 months during stable long-term treatment.
Chronic hypocalcemia — Initial	0.02–0.03 mg/kg	PO	Every 24 h	With oral calcium supplementation.
Chronic hypocalcemia — Maintenance	0.01–0.02 mg/kg	PO	Every 24–48 h

Indications

Maintenance treatment of hypocalcemia associated with primary hypoparathyroidism in dogs and cats.
Treatment of persistent hypocalcemia caused by hypoparathyroidism after thyroidectomy, particularly in cats.

Contraindications

Pregnancy
Hypercalcemia

Interactions

Calcium supplements and thiazides increase hypercalcemia risk.

Adverse effects

Hypercalcemia from excess dosing

Toxicity

Overdose causes hypercalcemia and tissue mineralization.

Preparations

Preparation	Form	Strength	Route
Dihydrotachysterol capsule 0.125 mg	Capsule	0.125 mg / capsule	PO
Dihydrotachysterol oral liquid 0.5 mg/mL Historical presentation contains 20% alcohol.	Oral Liquid	0.5 mg / mL	PO

Monitoring and Administration

Titrate to measured serum calcium; monitor renal function and distinguish total from ionized calcium.