Study findings indicate that generally well-functioning persons in their seventies with a TSH level falling in the mild subclinical hypothyroidism range (4.5–7.0 mU/L) do not have poorer functional mobility than their euthyroid counterparts. On the contrary, on the most demanding mobility parameters examined, these individuals appear to have a slight mobility advantage. Although persons in the moderate subclinical range (7.0–20.0 mU/L) had higher rates of perceived walking difficulty and contraindication to endurance activity in comparison to those in the euthyroid range, they demonstrated similar function on other mobility parameters. An examination of gait speed by finer categorizations of TSH level indicates that functional reserve increases with increasing TSH until 7.45 mU/L when it begins to decline. Remarkably, even in persons with TSH levels up to 10 mU/L rapid gait speed was statistically significantly faster than persons at the lowest level of TSH (.45–1.45 mU/L; all p<.001 except the second category where p=.011).
Few studies have examined functional status by thyroid function in older individuals. The Leiden 85-Plus Study18
found no association between TSH level and prevalence of disability in basic and instrument activities of daily living (ADL), however, those with higher TSH levels had less decline in instrumental ADL over four years. Even though measures of functional mobility are not directly comparable to ADL competence, findings from the current study are generally consistent with this work; in that, thyroid function was not found associated with functional deficits and when differences emerged persons with mildly elevated TSH levels demonstrated a functional advantage. The findings reported herein provide further support for a positive association between TSH level and functional independence and extends the association to a slightly younger group. Another study26
involving community-resident men aged 73 years and older in The Netherlands found greater grip strength and better lower-extremity performance in persons with higher FT4 levels. The association between TSH level and physical performance was not reported.
Even though findings indicate that older adults with subclinical hypothyroidism function as well as if not better than those with normal thyroid function, it is not known whether individuals with subclinical hypothyroidism who initiate treatment would experience improved or less decline in mobility. Based on medication data collected in Year 3, just under half (31 / 47.7%) of the 65 individuals with moderate subclinical hypothyroidism in Year 2 who were alive in Year 4 with complete follow-up data began new use of thyroid agonists. Examining gait speed decline, we found no difference between those who did and did not initiate treatment (27.7 vs. 18.9% p=.412 for usual and 44.4 vs. 24.2% p=.096 for rapid gait); however, the sample size is too small to make a definitive judgment. If anything, those initiating treatment tended to exhibit higher rates of decline.
It remains unclear if a mildly elevated TSH level directly contributes to better mobility status or reflects an underlying positive adaptation that fosters robust health. Recent observations of a right shift in the distribution of TSH with age1
and the apparent protective association with mortality in advanced age,18
have led to speculation that increased TSH secretion may be an adaptive response to an accumulation of thyroid antibodies that frequently occurs with age16
and thus may be a marker of pituitary resiliency and health. Others have suggested that a lower metabolic rate may protect against excessive catabolism.26
An important study limitation concerns incomplete assessment of thyroid function. As in most other observational cohort studies, 5, 8, 10
TSH level was measured on a single occasion. Given that acute stress, high physical activity and several pharmacologic agents can affect TSH level, a single high reading may represent a transient benign elevation.14, 16
FT4 was not measured in participants with a TSH between 4.5 and 6.9 mU/L, therefore some persons with overt hypothyroidism may have been included with the mild subclinical hypothyroidism group, which would tend to bias the findings towards poorer mobility function and underestimate any mobility advantage. In as much as casual assessment of TSH undertaken in a routine medical visit may serve as the basis for follow-up testing and possible treatment, the findings reported here suggest that for individuals in their seventies, a mildly elevated TSH level is no cause for concern. The small number of participants in the moderate subclinical hypothyroid group constitutes another limitation and thus findings of no difference between groups should be treated with caution.
Even though few studies have considered new treatment in examining differential outcomes by thyroid function over time,26
assessment of medication use over the follow-up period was limited to Year 3 only. Since participants received a report of their TSH levels after their Year 2 clinic visit, we would expect most new use to occur in Year 3 and as reported above new use was not found to impact rate of mobility decline. Therefore the lack of medication data from Year 4 does not appear to present a major shortcoming.
In summary, in generally well-functioning community resident persons in their seventies, mildly elevated TSH levels do not appear to indicate or confer health risks as reflected by multiple parameters of functional mobility. Despite the Institute of Medicine’s recommendation against routine TSH screening and thus treatment of asymptomatic subclinical hypothyroidism,26
controversy remains as to whether and when treatment should be initiated even in older adults.27
We believe the findings reported herein provide supportive evidence that mild to moderate elevations in TSH with normal FT4 pose little threat to the health and functioning of older adults. A better understanding of the meaning of the age-related increase in TSH level and its potential benefit warrants further study and consideration.