Hypothyroidism in patients older than 55 years: an analysis of the etiology and assessment of the effectiveness of therapy.

BACKGROUND: Several epidemiological studies on the prevalence of hypothyroidism in the elderly have been reported; however, extensive series of elderly patients with thyroid dysfunction have not been studied. Our aim has been to assess the relative frequency of the diverse causes of hypothyroidism in a group of patients older than 55 years and the adequacy of control of thyroid function attained by levothyroxine therapy.
METHODS: We performed a descriptive, observational, cross-sectional study in the setting of a hospital endocrinology clinic. From a total of 1581 patients older than 55 who were complaining of a thyroid disorder, we studied a group of 655 patients with hypothyroidism. There were 559 women (85.3%, age 65.01 +/- 7.90 years) and 96 men (14.7%, 65.36 +/- 8.39 years). In every patient, we collected etiology, presence of goiter, time of evolution from diagnosis and from therapy prescription, previous and present treatments, current thyroid functional status (free thyroxine and thyrotropin concentration), adequacy of disease control, and thyroid autoimmune status.
RESULTS: The causes of hypothyroidism were as follows: autoimmune thyroiditis, 308 (47.0%); postoperative hypothyroidism, 175 (26.7%); therapy for previous thyrotoxicosis, 63 (9.6%); thyrotropin deficiency, 15 (2.3%); iodine excess, 6 (0.9%); subacute thyroiditis, 2 (0.3%); and unknown etiology, 86 (13.1%) patients. Most patients with autoimmune thyroiditis were positive for thyroid peroxidase antibodies at the time of the study (94.4%). Mean (+/- SD) age at diagnosis was 61.8 +/- 9.4 years in men and 59.8 +/- 9.7 years in women. Median (range) duration of hypothyroidism was 1.4 (0-18) years in men and 3 (0-45) years in women ( p <.05). Adequacy of therapy was studied in 385 patients treated with replacement doses of levothyroxine. Two hundred and sixty (67.5%) of these subjects attained good control, whereas 125 (32.5%) showed inadequate control of the disease at the time of the study. A model of logistic regression showed that adequacy of therapy was dependent on the duration of therapy, but independent of age, gender, degree of hypothyroidism, etiology, autoimmune status, age at diagnosis, and dose of levothyroxine. A 2-year follow-up study performed in 56 newly diagnosed patients showed that an adequate control of hypothyroidism was attained in 35 (62.5%) patients at 6 months, in 46 (82.1%) patients at 1 year, and in 49 (87.5%) at 2 years of therapy with levothyroxine.
CONCLUSIONS: Autoimmune thyroiditis and postoperative hypothyroidism are the main causes of thyroid hypofunction in patients older than 55 years. The time from starting therapy is the main determinant of the adequacy of control of thyroid hypofunction in this population. With effective therapy and appropriate monitoring, more than 80% of the patients showed adequate control within 1 year of follow-up.