'Subclinical Hypothyroidism' Natural Course of the Syndrome During a Prolonged Follow-up Study

doi:10.1001/archinte.1993.00410080025004

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Vol. 153 No. 8, 26 APR 1993

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'Subclinical Hypothyroidism'

Natural Course of the Syndrome During a Prolonged Follow-up Study

Udaya M. Kabadi, MD

Arch Intern Med. 1993;153(8):957-961.

Abstract

Objective
To determine the natural course of the syndrome "subclinicalhypothyroidism."

Design
Prospective study of 30 subjects with "subclinical hypothyroidism,"as documented precisely by normal serum thyroxine and triiodithyronineconcentrations and high serum thyrotropin levels on three occasionsat intervals of 2 to 3 weeks. The subjects were followed upfor 4 to 15 years (mean, 8.2±2.3 years), with repeateddeterminations of thyroid hormone indices at intervals of 3to 6 months.

Setting
Endocrinology Clinic at the Veterans Affairs Medical Center.

Results
16 subjects developed definitive primary hypothyroidism within3 months to 2 years, as reflected by a progressive rise in serumthyrotropin level with a gradual decline in serum thyroxineand triiodithyronine concentrations, with serum thyroxine levelsfalling to subnormal concentrations. In 14 of these subjects,primary hypothyroidism could be attributed to known etiologicfactors, whereas in the remaining two the cause was not apparent.Persistently elevated serum thyrotropin with normal serum thyroxineand triiodithyronine concentrations following a cyclic patternwas observed in 14 subjects during the follow-up period. In11 of these subjects, there was a history of non-radical surgeryor conventional radiation therapy to the neck area, whereasin the remaining three subjects, no apparent cause could beidentified.

Conclusions
"Subclinical hypothyroidism" is not always a forerunner of primaryhypothyroidism. Two distinct populations evolve: (1) those withtrue preclinical or subclinical hypothyroidism, which may bepredicted by the presence of one of the well-known etiologicfactors responsible for onset of primary hypothyroidism, and(2) euthyroidism with reset thyrostat—a permanent statewithout a definitive progression to hypothyroidism, most probablysecondary to a previous subtle insult to the thyroid gland.

(Arch Intern Med. 1993;153:957-961)

Author Affiliations

From the Veterans Affairs Medical Centers, Des Moines, Iowa, and Phoenix, Ariz; and the Schools of Medicine, University of Iowa, Iowa City, and University of Arizona, Tucson.

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