The thyroid reserve in patients with breast cancer.

Patients with breast cancer were studied with regard to changes in the levels of serum thyroxine (T4) and serum triidothyronine (T3) after intramuscular administration of TSH for three days to investigate the thyroid reserve in these patients. The changes were examined also in normal subjects, in patients with Hashimoto's thyroiditis and primary hypothyroidism. In primary hypothyroidism no increase was seen in serum T4 and T3 levels after TSH administration, and the thyroid reserve was considered to be absent. In Hashimoto's thyroiditis the serum T3 levels after TSH administration were the same as in normal subjects, but the serum T4 levels were significantly lower than normal. This suggests that in patients with Hashimoto's thyroiditis the thyroid reserve is decreased and is in a state of so-called low thyroid reserve. In patients with breast cancer, however, the levels of serum T4 and T3 showed no difference as compared with those in normal subjects after TSH administration, and this implies that the thyroid reserve is well maintained.