A Suppressed Renin and a New Antihypertensive: Titrating Fludrocortisone in an Elderly Patient
A single patient whose renin-guided fludrocortisone titration collides with a second medication that suppresses renin on its own, entirely apart from his adrenal regimen.
Walter H., a 72-year-old retired mail carrier, still walks his old route most mornings out of habit more than necessity, though he has started stopping partway to catch his breath in a way he insists is "just age." He has had autoimmune Addison's disease for over thirty years, stable for most of it on hydrocortisone and fludrocortisone 0.1 mg daily, with plasma renin activity checked roughly yearly and kept, per the Endocrine Society's 2016 guideline, in the upper-normal reference range rather than fully suppressed. Two changes arrived at the same visit: his home blood pressure log shows a new average around 148/88, up from a stable 122/76 for years, and his most recent renin came back essentially undetectable — the reading that specific guideline warns against, since a fully suppressed renin usually signals mineralocorticoid over-replacement rather than adequate dosing.
The complication is that his primary care physician started him on hydrochlorothiazide eight weeks ago for that same new blood pressure, before this visit's renin was drawn — and thiazides independently suppress renin by their own diuretic mechanism, entirely apart from anything fludrocortisone is doing. At the same visit, he separately mentions that he has felt lightheaded standing up from his recliner a few times over the past month, always in the late afternoon, never bad enough that he actually sat back down or called anyone about it, which is part of why it took this long to come up. Read alone, that symptom points toward under- rather than over-replacement — the opposite direction from what his renin and blood pressure together seem to be arguing. He lives alone since his wife passed two years ago, which is part of why the geriatrician in the room is unwilling to treat an unwitnessed near-fall as a detail to revisit later. Two readings, both real, are pointing in opposite directions, and at least one of them may not mean what it normally means.
Follow-up visit, reviewing a suppressed renin
The Endocrine Society's own target is renin in the upper-normal range, not suppressed, and his is essentially undetectable while his blood pressure has climbed twenty-six points systolic. That's the textbook over-replacement picture. I'd reduce his fludrocortisone and recheck in a month.
I'm less comfortable reducing his mineralocorticoid on that number alone. He's telling us, unprompted, that he's felt lightheaded standing up — in a 72-year-old, that's not a symptom to talk him out of, it's a fall waiting to happen. If we're wrong about why his renin is suppressed and we cut his fludrocortisone anyway, we could turn an occasional dizzy spell into a real syncopal event.
You're both reading a real signal, but I don't think either signal is clean yet. Hydrochlorothiazide suppresses renin on its own, through volume depletion and distal tubule sodium handling entirely separate from mineralocorticoid receptor status — that was started eight weeks ago, right before this renin was drawn. We may be reading a thiazide effect and calling it fludrocortisone over-replacement.
Rather than adjust his fludrocortisone off a confounded number, I'd reassess whether the thiazide is even still the right choice for someone whose blood pressure rise could partly be from his adrenal regimen in the first place, and get a renin reading with that variable controlled before either of you moves his dose.
Agreed: switch his antihypertensive from hydrochlorothiazide to amlodipine, leave fludrocortisone unchanged, and repeat plasma renin activity in four weeks with the confounding variable removed.
Not agreed: what the fludrocortisone dose should do once that clean reading comes back suppressed, if it still does. The endocrinologist expects to reduce it regardless of the thiazide switch, reading his blood pressure trend as independently informative; the geriatrician wants the orthostatic symptom re-examined on its own before any dose change, regardless of what the renin shows next.