Increased risk of levothyroxine-treated hypothyroidism preceding the diagnosis of rheumatoid arthritis: a nationwide registry study.

OBJECTIVES

To evaluate the prevalence of levothyroxine-treated hypothyroidism in rheumatoid arthritis (RA) patients at the time of RA diagnosis in comparison to age- and sex-specific general population. Other objectives were to determine whether the risk of hypothyroidism varies by age at the onset of RA, or by sex or rheumatoid factor (RF) status.

METHODS

We identified 7,209 incident RA patients diagnosed between January 2004 and December 2007 from a Finnish nationwide register of special reimbursements for medication costs. The presence of hypothyroidism at RA diagnosis was identified from the same register based on special reimbursement decisions for levothyroxine substitution. The prevalence of levothyroxine-treated hypothyroidism was compared to that of an age- and sex-specific Finnish population, and a standardised rate ratio (SRR) for hypothyroidism was calculated.

RESULTS

The SRR for levothyroxine-treated hypothyroidism preceding RA was 1.51 (95% CI 1.35 to 1.67). Neither RF status nor sex modified the risk, although the results did not reach statistical significance among men. The SRR was highest, almost 2.5 among younger female RA patients (20-49 years of age), the excess prevalence of hypothyroidism decreasing steadily and wearing off among patients who were older at the time of diagnosis. The absolute prevalence of hypothyroidism, however, increased with age as it does in the general population.

CONCLUSIONS

The risk of hypothyroidism is increased among RA patients already at the disease onset, especially among the young women, regardless of RF status. This calls for attention to screening for hypothyroidism in RA patients, preferably when RA has already been diagnosed.