Effects of Elevated Triiodothyronine on Cognitive Performance and Mood in Healthy Subjects

Abstract

The acute effects of experimentally elevated triiodothyronine (T3) serum levels were investigated in 14 healthy male subjects. After oral application of 100 µg triiodothyronine on 3 consecutive days a battery of neuropsychological tests and rating scales for mood and bodily complaints were administered. Results show slight mood disturbances but no cognitive impairment caused by T3 level changes. Time intervals were estimated as being longer than in euthyroid state, and word production showed a trend to accelerate. It is concluded that the immediate effects of elevated circulating T3 on cognition and mood are merely discrete. More severe cognitive impairments reported in hyperthyroid patients are probably due to long-term effects on the brain.

Effect of Mild Thyrotoxicosis on Performance and Brain Activations in a Working Memory Task

“…To our knowledge there have been only three further studies analyzing the effect of experimentally raised thyroid hormones on cognitive performance. The first study of Kathmann et al [ 65 ] (i) shows no significant change in memory, attention or visuomotor coordination, but a trend towards an improved verbal fluency after the administration of 100 μg L-thyroxin per day for 3 days to 14 young euthyroid healthy men (20–37 years old). (ii) In the study of Baethge et al [ 66 ] the administration of 500 μg/d for 45 days on average to 11 subjects (4 men, 7 women, mean age 37 years old) did neither change attention, memory, visuospatial organization, verbal learning or verbal fluency.…”

“…It therefore cannot be concluded that externally administered thyroid hormones lead to “cognitive deficits”–on the contrary. The question remains, whether the cognitive deficits that long-term hyperthyroid patients experience are a result to their hyperthyroid state—it can be argued that a short-term hyperthyroid state may improve cognitive functions [ 65 – 67 ], while a long-term hyperthyroid state for years—besides to other diseases that are linked to hyperthyroidism [ 71 ]–will eventually lead to cognitive deficits.…”

Thyroid Function and Cognition during Aging

“…As shown in Table 5, the most common cognitive deficits observed in hyperthyroid patients include poorer performance on tests of attention, memory, mental alertness, and visuomotor speed [49, 51–54]. There are conflicting results concerning effects of hyperthyroidism on motor speed [55, 56]. Treatment of hyperthyroidism is followed by a partial return to baseline cognitive functioning [57].…”

“…Increasing serum T3 levels to values within the hyperthyroid range by administrating 100 μ g T3 daily over 3 days to 14 young, euthyroid healthy men (20–37 years old) resulted in no significant change in either attention, memory, or visuomotor coordination, but a trend towards improved verbal fluency was seen [56]. Administration of supraphysiologic doses of L-thyroxine (500 μ g/d for 45 days on average) to 11 subjects (4 men, 7 women, mean age 37 years) did not change attention, memory, visuospatial organization, verbal learning, or verbal fluency [59].…”

“… (a) Whybrow et al [49], MacCrimmon et al [51], Alvarez et al [52], Trzepacz et al [53], Toft [54]; (b) Kathman et al [56]; (c) Zeitlhofer et al [55]; (d) Münte et al [58]; (e) Baethge et al [59]. …”