Marked hyperprolactinemia in subclinical hypothyroidism.

Hyperprolactinemia is common in primary hypothyroidism but, to our knowledge, marked elevation of serum prolactin in subclinical hypothyroidism has not been previously reported. A 45-year-old woman presenting with carpal tunnel syndrome was found to have a minimally elevated level of thyrotropin with a normal level of thyroxine. Thyrotropin releasing hormone stimulation testing revealed hyperresponsiveness consistent with primary hypothyroidism. An unstimulated prolactin level was 187 micrograms/L and returned to normal with levothyroxine therapy. Results of visual field testing were normal, and a computed tomographic scan of the pituitary gland revealed no evidence of a macroadenoma. This case demonstrates the occurrence of significant hyperprolactinemia in the absence of overt primary hypothyroidism.