Exaggerated TSH responses to TRH in depressed patients with "normal" baseline TSH

Subclinical hypothyroidism (elevated thyroid-stimulating hormone [TSH] with normal thyroid hormone levels) can present with depression. This may be confirmed by an exaggerated TSH response to thyrotropin-releasing hormone (TRH) on the TRH stimulation test (TRH-ST). The objective of this study was to determine the prevalence of exaggerated TRH-ST results in a sample of depressed patients with "high-normal" screening TSH levels.

Depressed patients with TSH levels of 3.00-5.50 mIU/L underwent a TRH-ST. After baseline TSH was drawn, TRH 400 micrograms was injected intravenously, and TSH samples were drawn at +20 min, +30 min, and +40 min postinjection. A rise in TSH after TRH (peak value minus baseline) of > 25 mIU/L represented an exaggerated TSH response.

Twenty-three (38%) of 60 patients had an exaggerated TSH response to TRH. The 38% prevalence is significantly (Chi 2 = 59.65, df = 1, p < .001) greater than the 6% prevalence of positive TRH-ST results reported in the euthyroid general population. The prevalence of positive TRH-ST results was not attributable to differential patterns of psychotropic or thyroid hormone treatment. Unexpected observations were a lack of correlation in TSH levels week to week (r = .17, N.S.) and a lack of correlation between screening TSH value and subsequent TRH-ST results (r = .28, N.S.).

Subtle thyroid underfunction may be contributing to depression in some patients with TSH in the upper half of the range usually considered normal. If so, then the TRH-ST may be more sensitive in identifying this than measurement of TSH alone.