Serum thyroglobulin, TSH, thyroid hormones and thyroid volume were investigated during the menstrual cycle in 10 healthy females (day 2, 9, 16, 23 and day 2 of next cycle), during pregnancy (week 18, 24, 30 and 36) and post partum (1, 2, 3, 6 and 12 months) in 20 healthy females. During the menstrual cycle median serum thyroglobulin increased from 27 (day 2) to 32 micrograms/l (day 23, p less than 0.01). Serum TSH and thyroid volume demonstrated a similar increase with a positive correlation between serum thyroglobulin and thyroid volume (r = 0.65, p less than 0.02). Median serum thyroglobulin was significantly increased during the whole pregnancy (week 36, 73 micrograms/l) compared with post partum (1 month post partum, 22 micrograms/l, p less than 0.01), as was thyroid volume. Serum TSH was unaltered and free thyroid hormone indices decreased during pregnancy compared with post partum. No relation between changes in serum thyroglobulin and thyroid volume, TSH or thyroid hormones could be demonstrated. Serum thyroglobulin alterations thus were related to alterations in TSH and thyroid volume during the menstrual cycle. However, the increase in serum thyroglobulin and thyroid volume during pregnancy were unrelated to changes in serum TSH, indicating other mechanisms of regulation than TSH. When interpreting serum thyroglobulin levels in women, the co-existence of pregnancy and time of menstrual cycle should be taken into account in order to avoid misinterpretation of results.