Too Much of a Good Thing: Subclinical Hyperthyroidism vs Mortality & Heart Disease

As a family physician, I practice Goldilocks medicine.  You remember Goldie, complaining that one porridge was too hot and another too cold.  She also complained about one chair being too small while another was too large.  And in the final straw, she complained that one bed was too soft while another was too hard.  In the end, she insisted on the porridge, chair & bed that were just right (at least for her).

Likewise, when it comes to medicine, while reference ranges are good for comparison purposes and guidelines are useful for, well, guiding, my final goal is always a positive clinical effect in the patient's favor, rather than some numeric value.  In a variation on a theme, I was reminded of this upon reviewing a study published online earlier this week prior to print in the Archives of Internal Medicine in which the authors concluded that subclinical hyperthyroidism was associated with an increase risk of all-cause mortality, cardiovascular mortality & atrial fibrillation, with even greater cardiovascular mortality & atrial fibrillation especially when thyrotropin (TSH) was suppressed to less than 0.1mIU/L.

In order to arrive at their conclusion, the authors pooled data from 10 cohorts involving 52,67 participants, including 2,188 who had subclinical hyperthyroidism defined as TSH < 0.45mIU/L (euthyroid is 0.45-4.49mIU/L) w/normal free thyroxine (FT4) & triiodothyronine (FT3), all off exogenous thyroid medications.  In comparison, (clinical) hyperthyroidism is defined as TSH <0.45mIU/L w/elevated FT4 and/or FT3.  Upon analysis, the total cohort consisted of 58% women w/avg 58yo and were followed for just under 9yrs.

Because this study is only observation, one can only conjure up hypotheses as a result of correlation.  However, despite there being no demonstration of cause & effect, I am concerned that a similar risk profile will result from exogenous levothyroxine prescribed to such an effect.  Certainly, we have no proof to support my worries, as noted by the authors, but one cannot help but theorize that too much of a good thing may not be so.

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