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A specially designed method for measuring infant mortality among families with subsequent siblings of sudden infant death syndrome (SIDS) victims in the state of Washington, 1969 to 1984, yielded results similar to those from an earlier study in Norway. In both studies the SIDS rates among siblings were substantially lower than prior estimates. The rate of SIDS in siblings of infants who died of SIDS did not differ significantly from the SIDS rate among control infants matched for maternal age and birth order. Total infant mortality rates in the two groups were virtually identical. From the data at hand, it appears that earlier estimates of the risk of SIDS in siblings were inflated and that parents of SIDS victims can be counseled accordingly.

Serum triiodothyronine concentrations in victims of sudden infant death syndrome, when compared with those of both living and dead controls, were found to be elevated to a degree comparable to those found in a previous study. Thyroxine, thyroid-stimulating hormone, and thyroglobulin values were not elevated. Neonatal triiodothyronine and thyroxine values, determined on specimens collected within a few days of birth for most of the sudden infant death syndrome victims, did not differ significantly from those of comparable peers in their birth cohort. These results indicate that hypertriiodothyroninemia may serve as a useful postmortem diagnostic marker for the syndrome but not as a premortem predictor. Parenthetically, thyroid hormones appear to remain stable in either a frozen or dried state for up to two years.

Knowledge of the likelihood of a repetition of sudden infant death syndrome within a sibship, particularly in the next sibling, is important to parents. Methodologic considerations with respect to the studies of recurrence risk already published indicate that the rates reported are overestimates. This suspicion is confirmed by our study based on 826,162 infants surviving the first week of life on file in the Medical Birth Registry of Norway. A total of 1062 (1.3 per 1,000) infants died of SIDS; five deaths occurred as the second case in a family. The recurrence risk for the nextborn sibling was 5.6 per 1000, and for all subsequent siblings 4.8 per 1000, which would seem encouragingly low from a counseling point of view.