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Thyroid Research Recently Published Hypothyroidism

Subclinical hypothyroidism may disappear or progress to overt hypothyroidism

(November 2004)

The background of the study. Subclinical hypothyroidism, defined as a high serum thyrotropin (TSH) and a normal free thyroxine (T4) concentration, is common. It can disappear, but on the other hand it can progress to overt hypothyroidism (high serum TSH and low free T4 concentrations). In this study a group of patients with subclinical hypothyroidism was followed to determine the frequency of improvement or worsening.

How the study was done. The study subjects were 107 patients (93 women, 14 men; age range, 55 to 83 years) with subclinical hypothyroidism detected initially as an incidental finding and confirmed by repeat study 1 to 3 months later. Forty-four percent had one or more symptoms and signs of hypothyroidism, and 76 percent had a high serum antithyroid peroxidase (anti-TPO) antibody concentration. Seventy-five patients (70 percent) had other medical problems, such as diabetes mellitus or hypertension. The patients’ clinical status was evaluated and serum TSH and free T4 were measured at six-month intervals. Patients who had a serum TSH concentration ≥20 mU/L or a low serum free T4 concentration during follow-up were considered to have overt hypothyroidism and were treated with T4.

The results of the study. During a mean follow-up period of 32 months, serum TSH concentrations became normal in 40 patients (37 percent), and 28 patients (26 percent) had progression to overt hypothyroidism and were treated with T4 (Table). Normalization of TSH secretion was more likely to occur in those with lower base-line serum TSH values, and overt hypothyroidism was more likely to occur in patients with higher base-line serum TSH values (Table).

Table. Factors Affecting Outcome in Patients with Subclinical Hypothyroidism.
Overt Hypothyroidism Incidence Rate
(per patient-year)
Subclinical Hypothyroidism Normal Serum TSH
All (n = 107) 28 (26%) 0.09 39 (36%) 40 (37%)
Base-line serum TSH (mU/L)
5.0–9.9 mU/L
(n = 71)
4 (6%) 0.02 30 (42%) 37 (32%)*
10.0–14.9 mU/L
(n = 15)
6 (40%) 0.20 7 (47%) 2 (13%)*
15.0–19.9 mU/L
(n = 21)
18 (86%) 0.73 2 (9%) 1 (5%)

The conclusions of the study. Among patients with subclinical hypothyroidism, normalization of serum TSH values is common in those with serum TSH values of 5.0 to 9.9 mU/L, and progression to overt hypothyroidism is common in those with serum TSH values of 15.0 to 19.9 mU/L.

The original article. Diez JJ, Iglesias P. Spontaneous subclinical hypothyroidism in patients older than 55 years: an analysis of natural course and risk factors for the development of overt thyroid failure. J Clin Endocrinol Metab 2004;89:4890-7.

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