Demographics of participants
A total of 566 participants with plasma TSH measurement at baseline (Figure 1). One participant with plasma TSH values lower than the LDD was identified as an outlier and was excluded from analyses. And 89 participants who had the using record of thyroid medications, whether anti-thyroid medications or thyroid hormone replacement, at baseline or during follow-up were excluded from analyses also. Finally, 476 participants were included in this study comprising 49 cognitively normal (CN), 336 mild cognitive impairment (MCI), and 91 AD. Only three participants with baseline TSH values below the reference range, and 25 participants with baseline TSH values above the reference range. Since few participants completed the Aβ-PET scan at baseline, we used the Aβ-PET data obtained 12 months after baseline and TSH measurement at the same visit with Aβ-PET to investigate the cross-sectional association between TSH and Aβ-PET. All participants had covariates data available.
After 718.0 person-years of follow-up (median 2.0, IQR 2.0 years), 160 MCI converted to AD, and 156 MCI did not convert to AD (14 participants without one visit and 6 participants with MCI converted to other forms of dementia were excluded from analyses, Table 2). There were no differences in baseline TSH levels between participants stratified by baseline diagnosis or MCI conversion (Table 1-2).
Table 1 Baseline demographics of participants
|
CN (n = 49)
|
MCI (n = 336)
|
AD ( n= 91)
|
P
|
Age
|
73.10 (7.60)
|
75.1 (9.78)
|
75.30 (11.25)
|
0.765
|
Sex:Female, No.(%)
|
22 (44.9)
|
104 (31.0)
|
36 (39.6)
|
0.073
|
Education (years)
|
16.00 (4.00)
|
16.00 (4.00)
|
16.00 (5.00)
|
0.238
|
≥1 APOE ε4, No.(%)
|
4 ( 8.2)
|
183 (54.5)
|
64 (70.3)
|
<0.001
|
ADAS-Cog13
|
10.00 (5.66)
|
18.33 (8.66)
|
28.67 (11.67)
|
<0.001
|
MMSE
|
29.00 (1.00)
|
27.00 (2.00)
|
24.00 (3.00)
|
<0.001
|
CDR-SB
|
0.00 (0.00)
|
1.50 (1.00)
|
4.00 (1.50)
|
<0.001
|
CSF Aβ42 (pg/mL)
|
1458.00 (408.75)
|
666.20 (464.75)
|
547.60 (299.35)
|
<0.001
|
CSF t-tau (pg/mL)
|
205.75 (76.30)
|
288.90 (147.10)
|
345.30 (169.00)
|
<0.001
|
CSF p-tau181 (pg/mL)
|
17.49 (6.45)
|
27.78 (17.66)
|
33.57 (17.25)
|
<0.001
|
Hippocampus (mL)
|
7.32 ± 0.83
|
6.42 ± 1.08
|
5.85 ± 1.15
|
<0.001
|
Ventricles (mL)
|
30.90 (20.68)
|
39.94 (30.20)
|
45.40 (32.67)
|
<0.001
|
Whole-brain (mL)
|
1002.98 ± 98.06
|
1003.49 ± 111.81
|
973.31 ± 124.51
|
0.078
|
FDG-PET
|
1.31 ± 0.11
|
1.19 ± 0.13
|
1.07 ± 0.14
|
<0.001
|
Aβ-PET*
|
1.31 (0.11)
|
1.98 (0.72)
|
1.87 (0.38)
|
0.069
|
TSH (mIU/L)
|
2.10 (1.30)
|
1.80 (1.40)
|
1.90 (1.15)
|
0.495
|
*Measured at 12 months after baseline.
Table 2 Demographics of participants stratified by MCI conversion*
|
MCI-nonconverter (n = 156)
|
MCI-converter ( n= 160)
|
P
|
Age
|
74.85 (9.75)
|
75.80 (9.63)
|
0.913
|
Sex:Female, No.(%)
|
39 (25.0)
|
58 (36.2)
|
0.041
|
Education (years)
|
16.00 (6.00)
|
16.00 (4.00)
|
0.205
|
≥1 APOE ε4, No.(%)
|
74 (47.4)
|
102 (63.7)
|
0.005
|
BMI
|
26.12 (4.35)
|
25.15 (4.71)
|
0.022
|
SBP
|
135.50 (27.00)
|
137.00 (22.50)
|
0.422
|
DBP
|
75.17 ± 9.53
|
74.42 ± 9.68
|
0.489
|
Diabetes, No.(%)
|
13 ( 8.3)
|
12 ( 7.5)
|
0.947
|
Hyperlipidemia, No.(%)
|
62 (39.7)
|
75 (46.9)
|
0.244
|
Depression scores
|
1.00 (1.00)
|
1.00 (1.00)
|
0.946
|
TSH (mIU/L)
|
1.90 (1.50)
|
1.70 (1.43)
|
0.424
|
*14 participants without one visit and 6 participants with MCI converted to other forms of dementia were excluded from analyses. BMI, body mass index; SBP, systolic blood pressure; DBP, diastolic blood pressure.
Associations between TSH and cognition and Alzheimer’s pathology
As shown in Table 3, we observed no significant associations between TSH and cognition and AD biomarkers (all p > 0.05). There were also no significant interactions between sex and TSH on these measures (all p for interaction > 0.05).
Table 3 Cross-sectional associations between TSH and other measures
|
n
|
B
|
95% CI
|
p
|
p for interaction
|
ADAS-Cog13
|
471
|
-0.28
|
-0.84, 0.28
|
0.333
|
0.165
|
MMSE
|
476
|
-0.07
|
-0.23, 0.09
|
0.41
|
0.530
|
CDR-SB
|
476
|
-0.02
|
-0.13, 0.09
|
0.731
|
0.143
|
CSF Aβ42
|
290
|
9.4
|
-24.78, 43.57
|
0.589
|
0.976
|
CSF t-tau
|
290
|
-3.7
|
-15.41, 8.03
|
0.536
|
0.304
|
CSF p-tau181
|
290
|
-0.58
|
-1.86, 0.71
|
0.378
|
0.224
|
Hippocampus
|
378
|
-0.01
|
-0.09, 0.07
|
0.739
|
0.179
|
Ventricles
|
465
|
-0.99
|
-2.32, 0.34
|
0.143
|
0.192
|
Whole-brain
|
468
|
1.05
|
-3.05, 5.16
|
0.615
|
0.555
|
FDG-PET
|
248
|
0.004
|
-0.01, 0.02
|
0.632
|
0.792
|
Aβ-PET*
|
61
|
-0.01
|
-0.10, 0.08
|
0.763
|
0.556
|
*Measured at 12 months after baseline.
Association between TSH and the risk of AD
We found no significant association or sex differences between TSH and the risk of conversion from MCI to AD in the full range of TSH (Table 4). However, we found that sex modified the TSH-associated risk of AD in the normal range of TSH (p for interaction = 0.043, model 3, Table 4). Sex-stratified analyses in the normal range of TSH seemed to indicate an increased risk of AD in male with higher levels of TSH (HR = 1.33, p = 0.077, model 3, Table 5). The association was not significant, which may be due to the smaller sample sizes. In analyses stratified TSH into tertiles, we found the highest TSH tertile significantly increased the risk of AD in female compared with the lowest TSH tertile within the normal range (HR = 2.62, p = 0.021, model 3, Table 5). TSH was not associated with AD risk within the full or normal range of TSH in male.
Table 4 Association between TSH and the risk of AD*
|
n
|
Model 1
|
Model 2
|
Model 3
|
HR
|
95%CI
|
P
|
P for interaction
|
HR
|
95%CI
|
P
|
P for interaction
|
HR
|
95%CI
|
P
|
P for interaction
|
TSH
|
316
|
1.00
|
0.89, 1.12
|
0.980
|
0.185
|
0.98
|
0.87, 1.10
|
0.725
|
0.205
|
0.98
|
0.87, 1.11
|
0.793
|
0.173
|
TSH within the normal range (0.45-4.49 mIU/l)
|
296
|
0.99
|
0.83,1.18
|
0.874
|
0.025
|
0.97
|
0.82,1.16
|
0.759
|
0.033
|
0.97
|
0.81,1.15
|
0.696
|
0.043
|
*Model 1: unadjusted. Model 2: adjusted for age, sex, and APOE ε4. Model 3: further adjusted for blood pressure, diabetes, hyperlipidemia, BMI, and depression scores.
Table 5 Association between TSH and AD risk within the normal range stratified by sex*
|
n
|
Model 1
|
|
Model 2
|
|
Model 3
|
HR
|
95%CI
|
P
|
|
HR
|
95%CI
|
P
|
|
HR
|
95%CI
|
P
|
Continuous TSH
|
Male
|
209
|
0.84
|
0.67,1.07
|
0.159
|
|
0.83
|
0.66,1.05
|
0.123
|
|
0.84
|
0.67,1.06
|
0.149
|
Female
|
87
|
1.28
|
0.97,1.69
|
0.076
|
|
1.26
|
0.95,1.66
|
0.108
|
|
1.33
|
0.97,1.83
|
0.077
|
Tertiles of TSH
|
Male
|
Lowest
(0.45-1.40mIU/L)
|
77
|
Reference
|
Middle (1.50-2.10mIU/L)
|
64
|
1.05
|
0.67,1.66
|
0.824
|
|
1.06
|
0.67,1.67
|
0.81
|
|
1.10
|
0.69,1.77
|
0.688
|
Highest (2.20-4.30mIU/L)
|
68
|
0.68
|
0.41,1.14
|
0.142
|
|
0.67
|
0.40,1.12
|
0.123
|
|
0.71
|
0.42,1.20
|
0.199
|
Female
|
Lowest (0.46-1.30mIU/L)
|
31
|
Reference
|
Middle (1.40-2.30mIU/L)
|
28
|
1.76
|
0.89.3.47
|
0.104
|
|
1.63
|
0.82,3.26
|
0.164
|
|
2.26
|
1.03,4.96
|
0.042
|
Highest (2.40-4.20mIU/L)
|
28
|
1.93
|
0.98,3.79
|
0.058
|
|
1.91
|
0.97,3.77
|
0.062
|
|
2.62
|
1.16,5.94
|
0.021
|
*Model 1: unadjusted. Model 2: adjusted for age, sex, and APOE ε4. Model 3: further adjusted for blood pressure, diabetes, hyperlipidemia, BMI, and depression scores.
Sensitive analyses
Stratified by baseline diagnosis analyses still suggested no correlations between TSH with cognition and AD biomarkers in each group (Additional file, Table 1-3). There were also no correlations between the above measures and TSH within the normal range (Additional file, Table 4).