From the article:
Sex hormones are thought to play a part in the development of rheumatoid arthritis, and both men and women with the condition tend to have lower levels of testosterone in their blood than healthy people. But it is not clear whether this is a contributory factor or a consequence of the disease.
The researchers based their findings on participants of the Swedish Malmo Preventive Medicine Program (MPMP), which began in 1974 and tracked the health of more than 33,000 people born between 1921 and 1949.
[…]
After taking account of smoking and body mass index, both of which can affect the risk of rheumatoid arthritis, **men with lower levels of testosterone in their blood samples were more likely to develop the disease.
This was **statistically significant for those who tested negative for rheumatoid factor when they were diagnosed.