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Low serum free thyroxine index in ambulating elderly is due to a resetting of the threshold of thyrotropin feedback suppression.
J Clin Endocrinol Metab. 1991 Oct; 73(4):843-9.JC

Abstract

Subnormal free T4 index (FT4I) values (less than 80) with inappropriately normal serum TSH concentrations that could not be attributed to illness or drugs were found in 2.5% of ambulating elderly clinic patients. Six such individuals (three men and three women, aged 68.8 +/- 4.8 yr) were selected for their persistent thyroid test abnormalities and were sex and age matched to six subjects (67.7 +/- 4.9 yr) with normal FT4I (greater than 90) and TSH levels. The former also had low serum total T4 (TT4) and rT3 (TrT3) concentrations, but total T3 (TT3) and basal TSH values were normal and did not differ between the groups. Responses of ACTH, LH, FSH, TSH, and PRL to stimulation with CRH, GnRH, and TRH showed no differences between the two groups, indicating that the normal TSH concentration, inappropriate for the low FT4I level, is not due to generalized hypothalamic or pituitary dysfunction. Administration of 3 g iopanoic acid (IOP) daily for 3 days produced significant increases in the TT4 and TrT3 concentrations to the same degree in both groups. Also, in both groups the IOP-induced suppression of T4 to T3 conversion in the pituitary gland provoked similar increases in basal TSH (280 +/- 47% and 288 +/- 33%) and TSH secretion in response to TRH (173 +/- 7% and 156 +/- 13%). These results indicate that the low FT4I is not the consequence of reduced pituitary TSH reserve. In addition, evidence for normal thyroid gland reserve and the secretion of TSH of normal biological activity was obtained by comparing the acute iodothyronine responses to TRH-induced TSH release in both groups. It is concluded that the normal serum TSH concentration, inappropriate for the low FT4I value in some elderly subjects, is due to an apparent resetting of the thyroid hormone feedback regulation threshold of TSH secretion. It may, in turn, be the result of enhanced pituitary conversion of T4 to T3 or increased T4 uptake by the thyrotrophs.

Authors+Show Affiliations

Department of Medicine, University of Chicago, Illinois 60637.No affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

1653784

Citation

Lewis, G F., et al. "Low Serum Free Thyroxine Index in Ambulating Elderly Is Due to a Resetting of the Threshold of Thyrotropin Feedback Suppression." The Journal of Clinical Endocrinology and Metabolism, vol. 73, no. 4, 1991, pp. 843-9.
Lewis GF, Alessi CA, Imperial JG, et al. Low serum free thyroxine index in ambulating elderly is due to a resetting of the threshold of thyrotropin feedback suppression. J Clin Endocrinol Metab. 1991;73(4):843-9.
Lewis, G. F., Alessi, C. A., Imperial, J. G., & Refetoff, S. (1991). Low serum free thyroxine index in ambulating elderly is due to a resetting of the threshold of thyrotropin feedback suppression. The Journal of Clinical Endocrinology and Metabolism, 73(4), 843-9.
Lewis GF, et al. Low Serum Free Thyroxine Index in Ambulating Elderly Is Due to a Resetting of the Threshold of Thyrotropin Feedback Suppression. J Clin Endocrinol Metab. 1991;73(4):843-9. PubMed PMID: 1653784.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Low serum free thyroxine index in ambulating elderly is due to a resetting of the threshold of thyrotropin feedback suppression. AU - Lewis,G F, AU - Alessi,C A, AU - Imperial,J G, AU - Refetoff,S, PY - 1991/10/1/pubmed PY - 1991/10/1/medline PY - 1991/10/1/entrez SP - 843 EP - 9 JF - The Journal of clinical endocrinology and metabolism JO - J Clin Endocrinol Metab VL - 73 IS - 4 N2 - Subnormal free T4 index (FT4I) values (less than 80) with inappropriately normal serum TSH concentrations that could not be attributed to illness or drugs were found in 2.5% of ambulating elderly clinic patients. Six such individuals (three men and three women, aged 68.8 +/- 4.8 yr) were selected for their persistent thyroid test abnormalities and were sex and age matched to six subjects (67.7 +/- 4.9 yr) with normal FT4I (greater than 90) and TSH levels. The former also had low serum total T4 (TT4) and rT3 (TrT3) concentrations, but total T3 (TT3) and basal TSH values were normal and did not differ between the groups. Responses of ACTH, LH, FSH, TSH, and PRL to stimulation with CRH, GnRH, and TRH showed no differences between the two groups, indicating that the normal TSH concentration, inappropriate for the low FT4I level, is not due to generalized hypothalamic or pituitary dysfunction. Administration of 3 g iopanoic acid (IOP) daily for 3 days produced significant increases in the TT4 and TrT3 concentrations to the same degree in both groups. Also, in both groups the IOP-induced suppression of T4 to T3 conversion in the pituitary gland provoked similar increases in basal TSH (280 +/- 47% and 288 +/- 33%) and TSH secretion in response to TRH (173 +/- 7% and 156 +/- 13%). These results indicate that the low FT4I is not the consequence of reduced pituitary TSH reserve. In addition, evidence for normal thyroid gland reserve and the secretion of TSH of normal biological activity was obtained by comparing the acute iodothyronine responses to TRH-induced TSH release in both groups. It is concluded that the normal serum TSH concentration, inappropriate for the low FT4I value in some elderly subjects, is due to an apparent resetting of the thyroid hormone feedback regulation threshold of TSH secretion. It may, in turn, be the result of enhanced pituitary conversion of T4 to T3 or increased T4 uptake by the thyrotrophs. SN - 0021-972X UR - https://www.unboundmedicine.com/medline/citation/1653784/Low_serum_free_thyroxine_index_in_ambulating_elderly_is_due_to_a_resetting_of_the_threshold_of_thyrotropin_feedback_suppression_ L2 - https://academic.oup.com/jcem/article-lookup/doi/10.1210/jcem-73-4-843 DB - PRIME DP - Unbound Medicine ER -