Lack of acute zinc effect on thyrotropin-releasing hormone–stimulated thyroid-stimulating hormone secretion during oral zinc tolerance test in healthy men

It is known that zinc interacts with thyroid hormones, altering plasma thyroid-stimulating hormones (TSH), triiodothyronine (T3), and tetraiodothyronine (T4) in rats and humans. To investigate whether zinc affects thyrotropin-releasing hormone (TRH)–stimulated TSH, 5 healthy men were tested at 8:00 am after an overnight fast. Elemental zinc (37.5 mg, as heptahydrated zinc sulfate) was administered orally, and TRH (200 μg) was administered intravenously for 1 minute. Blood samples were collected from an antecubital vein at −30, 0, 30, 60, 90, 150, 210, 240, 270, and 300 minutes. Serum zinc remained constant throughout the control period. However, serum zinc was significantly elevated after oral zinc administration in the experimental group (P < .05). Plasma TSH levels increased after TRH injection in the control and in the experimental group, although not significantly when comparing both groups. Moreover, no significant areas under the curves and no correlation between serum zinc and plasma TSH levels were detected in both groups studied. This is the first study that showed the relationship between TRH-stimulated TSH secretion during oral zinc tolerance test in healthy men. These results suggest that a single oral dose of zinc did not change TSH secretion after TRH injection. Furthermore, acute increases in blood TRH and TSH did not alter serum zinc levels.