Primary Hypothyroidism with Exceptionally High Prolactin—A Really Big Deal

Background Primary hypothyroidism can cause both hyperprolactinemia and pituitary hyperplasia. The degree of hyperprolactinemia is generally modest, and rarely do prolactin concentrations exceed 100 ng/mL (4.34 nmol/L). This combination of hyperprolactinemia and pituitary gland enlargement might raise suspicion for a prolactinoma or a nonfunctioning adenoma limiting the ability of hypothalamic dopamine to inhibit prolactin production, the so-called “stalk effect.” Case Description We describe a 30-year-old female with headaches, galactorrhea, and hyperprolactinemia with a presumptive diagnosis of a prolactinoma. She had hypothyroidism after treatment of Graves disease at age 17 years but was noncompliant with levothyroxine replacement. Thyroid-stimulating hormone (TSH) was 263 mIU/L, FT4 was 3.01 pmol/L, and prolactin was 323 ng/mL (14.06 nmol/L). Magnetic resonance imaging (MRI) demonstrated increased volume of the pituitary gland with homogeneous enhancement with gadolinium. Levothyroxine treatment for 2 weeks decreased her TSH to130 mIU/L, but her total prolactin remained elevated at 386 ng/mL (16.78 nmol/L). Further testing identified that 76% of the total prolactin was macroprolactin. Conclusions Primary hypothyroidism can cause hyperprolactinemia, and prolonged disease may lead to pituitary hyperplasia. However, a marked elevation of prolactin should raise suspicion to investigate additional etiologies for hyperprolactinemia. Our case exemplifies a dual etiology for hyperprolactinemia and pituitary hyperplasia caused by both hypothyroidism and macroprolactin. This knowledge is invaluable for clinicians to avoid unnecessary management with dopamine agonists and/or surgery. This patient's prolactin was 323 ng/mL (14.06 nmol/L). Before our case, the highest prolactin in a hypothyroid patient reported in the literature was 253 ng/mL (11.0 nmol/L).