Summary
Nuclear binding of thyroxine (T4) and triiodothyronine (T3) in mononuclear blood cells was investigated in 12 young (age 16-30 years) healthy
subjects (group A), in 12 middle-aged (age 31-60 years) healthy subjects (group B)
and in 12 elderly (61-90 years) healthy subjects. Serum free T3 was depressed in group C as compared to the younger age groups, whereas serum free
T4 and TSH did not differ between the groups.
Maximal specific nuclear binding capacity for both T4 and T3 decreased with increasing age, T4 group A: 1.2 fmol T4/100 μg DNA, group B: 1.2 fmol T4/100 μg DNA, group C: 0.7 fmol T4/100 μg DNA; T3 group A: 1.7 fmol T3/100 μg DNA, group B: 1.0 fmol T3/100 μg DNA, group C: 0.9 fmol T3/100 μg DNA. The equilibrium association constant (Ka) for T4 increased with age, group A: Ka = 3.3×109 l/mol, group B: Ka = 3.2×109 l/mol, group C: Ka = 6.4×109 l/mol, whereas Ka for nuclear binding of T3 decreased with age, group A: Ka = 3.9 × 109 l/mol, group B: Ka = 5.9 × 109 l/mol, group C: Ka = 1.8 × 109 l/mol.
We conclude that, whereas the opposite variations of nuclear capacity and binding
affinity for T4 tend to preserve the nuclear T4 concentration, the nuclear T3 concentration definitely decreases with age. The unaltered serum levels of TSH suggest
that the decrease of both serum levels of free T3 and the nuclear T3 concentration might represent physiologically changes in old age.
Key-Words:
Old Age
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Thyroid Hormones
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Thyroxine Receptors
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Triiodothyronine Receptors
