Inhibited 131I Uptake but Normal Release of Thyroid Hormone by Thyroid Gland in Response to TSH Administration in Subclinical Hypothyroidism

Abstract

Background: Subclinical hypothyroidism (SH) is a syndrome characterized by normal thyroid hormone with supernormal TSH levels. In most subjects, a known etiologic factor is identified and therapy with levothyroxine normalizes serum TSH while maintaining normal thyroid hormone levels. However, the exact pathophysiology of these thyroid hormone alterations is not defined.Objective:A major step in synthesis i.e. iodine uptake and the release of thyroid hormones in response to administration of TSH were assessed in subjects with SH.Subjects and Methods: 10 men and 5 women with ages 42-76 years with SH secondary to Hashimoto's thyroiditis and 10 euthyroid men (N)with ages 39-70 years participated in the study. 24 hr 131 I thyroid uptake and serum TSH, T4 and T3 were determined before and after SC administration of recombinant human TSH, 0.9 mg for two consecutive days. Comparisons were conducted between subjects with SH and N. Results: In subjects with SH, 24 hour 131I thyroid uptakes were within normal range [10-30%] as defined by local nuclear medicine laboratory. However, the mean value (13(plus or minus)2)was significantly lower [p<0.05] as compared to N(25(plus or minus)4). 24 hr uptake values rose following TSH administration in both groups. However, the rise was significant [p<0.01] only in N. Furthermore, both the mean absolute uptake value and the mean % rise following TSH administration from preTSH uptake value were significantly lower in subjects with SH (16(plus or minus)3 and 22(plus or minus)3) in comparison to N(51(plus or minus)9 and 80(plus or minus)10).Serum T3 and T4 concentrations in subjects with SH were within normal range(80-180 ng/dl; 4.5-11.0 ug/dl) as established by the clinical laboratory. Moreover, the mean levels (136(plus or minus)11 and 8.3(plus or minus)0.7) were also not significantly different in comparison to N(141(plus or minus)12 and 8.4(plus or minus)0.6). Basal Serum TSH concentrations were supernormal and significantly higher[p<0.05] in subjects with SH(8(plus or minus)2)in comparison to N(2(plus or minus)1) and rose markedly in both groups following TSH administration with no significant difference among the elevations between groups. Serum T4 and T3 rose significantly from PreTSH levels in both groups (p<0.05). However, the rises were not significantly different between groups (T4 ug/dl,10.2(plus or minus)1.1in SH Vs10.4(plus or minus)1.2in N; T3 n g/dl, 166(plus or minus)17in SH vs.170(plus or minus)16 in N)Conclusion: In subjects with SH secondary to Hashimoto's thyroiditis, 24 hour 131 I Thyroid uptake is inhibited prior to as well as following SC TSH administration in comparison to N with maintainance of normal hormone release.