Complex alteration of thyroid function in healthy centenarians

Several changes in thyroid function have been described in the elderly and largely attributed to concomitant nonthyroidal illness. The extent to which aging per se contributes to these changes remains to be elucidated, and scanty data are available in extremely old subjects. The present study was designed to focus on thyroid function during physiological aging, taking advantage of two groups of selected aged individuals: group A of healthy centenarians (n = 41; age range, 100-110 yr) and group B including healthy elderly subjects selected by the criteria of the EURAGE SENIEUR protocol (n = 33; age range, 65-80 yr). Control groups included 98 healthy normal adult subjects (group C; age range, 20-64 yr) and 52 patients with miscellaneous nonthyroidal illness (group D; age range, 28-82 yr). Our previous report of a low prevalence of thyroid autoantibodies in centenarians was confirmed and extended by the finding of a similar low autoantibody prevalence in the highly selected healthy elderly population of group B. Subclinical primary hypothyroidism was found in 3 (7.3%) centenarians, and their data were excluded from further statistical evaluation. No significant difference was found in the median serum free T₄ levels of groups A-C. Median (and range) serum free T₃ (FT₃) was lower in centenarians [3.67 pmol/L (2.3-5.5)] than in group B [5.22 pmol/L (3.4-6.1)] and group C [5.38 pmol/L (2.9-8.4); P <0.0001 vs. both groups]. Similarly, the median serum TSH level of centenarians [0.97 mU/L (3 and TSH concentrations showed a significant inverse correlation (r = -0.634; P <0.0001 and r = -0.377; P <0.0001, respectively) with age. Median serum FT₃ in centenarians was lower than that in group D patients [4.61 pmol/L (2.15-6.6); P <0.0001]. In contrast, median serum rT₃ in centenarians [0.40 nmol/L (0.20-0.77)], although higher than those in groups B [0.24 nmol/L (0.15-0.37); P <0.0001] and C [0.22 nmol/L (0.05-0.46); P <0.0001], was significantly lower than that in group D [0.60 nmol/L (0.13-2.08); P <0.0001]. In conclusion, thyroid function appears to be well preserved until the eighth decade of life if healthy subjects are studied, whereas a reduction of serum FT₃ is observed in extreme aging. This phenomenon appears to be the consequence of both reduced thyroid activity, resulting from decreased serum TSH concentration, and impairment of peripheral 5′-deiodinase.