1. Serum calcium was measured under different circumstances of thyroid dysfunction in male Sprague-Dawley and female Wistar rats.
2. After induction of hypothyroidism with 131I and propylthiouracil, serum calcium fell from a control value of 2.43 ± 0.08 mmol/l to 2.07 ± 0.08 mmol/l (P < 0.01).
3. Thyroid hormone replacement therapy in hypothyroid rats caused a return of serum calcium to control values (2.09 ± 0.04 mmol/l to 2.41 ± 0.02 mmol/l, P < 0.01).
4. Treatment of normal rats with thyroid hormones caused elevation of serum calcium to 2.59 ± 0.04 mmol/l compared with a control value of 2.44 ± 0.02 mmol/l (P < 0.01).
5. Time-course studies demonstrated a slow decline in serum calcium levels over a 10 week period during induction of hypothyroidism with propylthiouracil alone but a very rapid normalization of serum calcium (within 1 h) after replacement therapy with l-tri-iodothyronine.
6. These data illustrate the considerable influence that thyroid hormones exert on calcium metabolism and demonstrate for the first time consistent changes in serum calcium in the rat with hypo- and hyper-thyroidism.