Scarring hair loss works differently from the shedding and thinning most people picture. The inflammation sits at the stem cell region of the follicle, and once that region is destroyed the hair does not grow back. Two of the most common forms are frontal fibrosing alopecia, which walks the front hairline backward and often takes the eyebrows with it, and lichen planopilaris, which shows up as patches across the scalp with redness and scale right at the follicle opening. Clinicians have been noting for years that patients with these two conditions often carry other diagnoses alongside them, and a group at the University of Córdoba and Reina Sofía University Hospital in Spain set out to count how often. They registered the plan in advance, searched MEDLINE, Embase and ClinicalTrials.gov from inception through June 18, 2025, and ended up with 60 observational studies published between 2009 and 2025, covering 4,499 patients with FFA and 1,370 with LPP. Then they pooled the figures separately for each condition. In FFA, the most commonly reported conditions were pattern hair loss at 31.9 percent, high blood pressure at 23.6 percent, thyroid disorders overall at 22.2 percent, an underactive thyroid specifically at 20.6 percent, lichen planus pigmentosus at 19.6 percent, and abnormal cholesterol at 17.9 percent. In LPP the lineup looked similar with different weights: pattern hair loss at 27.2 percent, high blood pressure at 21.2 percent, underactive thyroid at 18.7 percent, vitamin D deficiency at 18.3 percent, abnormal cholesterol at 16.0 percent, and thyroid disorders overall at 14.5 percent. They also split the FFA numbers by sex, where pattern hair loss ran 27.2 percent in women and 58.0 percent in men, underactive thyroid ran 26.0 percent in women and 6.9 percent in men, and rosacea ran 10.5 percent in women and 28.1 percent in men.

The authors spend a good share of the paper on one distinction, and it is the thing worth carrying out of this. What they measured is prevalence, meaning how often a condition turns up inside a group of people who already have FFA or LPP. What they did not measure is association, meaning whether that condition turns up more often than it would in a comparable group of people without either diagnosis. Those are two separate questions, and this paper only answers the first. They state it plainly: the findings should not be interpreted as evidence of excess risk or causal relationships. That matters because earlier reviews asking the second question came back quieter. This paper cites Phan and Smith finding no significant association between LPP and abnormal cholesterol or thyroid disease, and Chuang and colleagues finding no significant association between LPP and high blood pressure, diabetes, high cholesterol, obesity or heart failure. So the sentence “about one in five FFA patients has a thyroid condition” describes who walks into these clinics. Whether FFA causes thyroid disease, or raises anyone’s odds of it, is a different question this paper leaves open. The rest of the limitations pull the same direction. Study quality came out mostly moderate, scoring 5 to 8 on a nine-item appraisal checklist with most landing at 6 or 7, and the recurring weak points were whether the patients sampled represented the wider population, whether sample sizes were adequate, and missing response rates. Most of the underlying data came from retrospective, cross-sectional or case series records. Several pooled figures moved noticeably when any single study was dropped, and some of the confidence intervals are very wide: pattern hair loss in LPP is reported as 27.2 percent with a range running from 10.3 to 55.0 percent, which is close to saying somewhere between a tenth and a half. The included studies also came largely from specialist referral centers, where patients tend to be the more complicated ones and where more testing gets ordered, so part of what looks like a high rate may be the result of someone going looking. None of the attempts to explain the variation between studies survived correction for testing many things at once. The authors’ own conclusion stays deliberately modest: these numbers are a quantitative map for individualized clinical assessment, and they write explicitly that their findings do not support universal laboratory screening for every condition on the list.

Here’s the study: Sanz-Cabanillas JL, Gómez-García FJ, Sanz-Cabanillas EB, Gay-Mimbrera J, Vélez García-Nieto AJ, Isla-Tejera B, Ruano J. Comorbidity burden in frontal fibrosing alopecia and lichen planopilaris: a systematic review and prevalence meta-analysis. Journal of Clinical Medicine, 2026;15(19):7728. doi:10.3390/jcm15197728. Indexed on PubMed (PMID 42882189). Read the full text here.

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