Incidence rate and clinical features of hyperthyroidism in a long-term iodine sufficient area of Sweden (Gothenburg) 2003-2005

Abstract

Objectives: To study hyperthyroidism in long-term iodine sufficiency (IS), as iodine supply affects its occurrence.

Design: Prospective descriptive study.

Patients: In 2003-2005, all referred cases of subclinical (SH) and overt hyperthyroidism (OH) were registered at diagnosis from a population (n = 631 239) in Gothenburg, Sweden.

Measurements: Information on age, gender, smoking, thyroid associated ophthalmopathy (TAO), thyroid hormones and TSH receptor antibodies (TRab) was collected. Incidences were calculated. SH and OH cases with Graves' disease (GD), toxic multinodular goiter (TMNG) and solitary toxic adenoma (STA) were compared. In GD, TRab+ and TRab- cases and patients with (TAO+) and without TAO (TAO-) were compared.

Results: The total incidence (n/100 000/year) of hyperthyroidism was 27·6; OH 23·8; SH 3·8; GD 21·4; TMNG 4·3; and STA 1·8. SH was more common among TMNG (40·2%) and STA (45·7%) than in GD (5·9%). SH-GD patients were older, more often smokers and had lower TRab levels than OH-GD patients. FreeT4 and T3 levels in GD were higher than in TMNG and STA. FreeT4, T3 and TRab decreased with age in patients with GD, P < 0·0001. TRab- patients had lower T3 than TRab+ patients, P < 0·001. TRab was positively correlated to FreeT4, P < 0·0001. TAO occurred in 20% of patients with GD. TAO+ patients were younger than TAO- patients. Smokers did not have more TAO.

Conclusion: The total incidence of hyperthyroidism was low. GD dominated with an age-related decline of thyroid hormones and TRab levels. The spectrum of hyperthyroidism in this long-term IS area may represent the future situation for countries with shorter history of IS.