Background: Many physiological effects of thyroid hormone (TH) are mediated by its canonical action via nuclear receptors (TH receptor a and b [TRa and TRb]) to regulate transcription of target genes. Heterozygous dominant negative mutations in human TRa mediate resistance to thyroid hormone alpha (RTHa), characterized by features of hypothyroidism (e.g., skeletal dysplasia, neurodevelopmental retardation, constipation) in specific tissues, but near-normal circulating TH concentrations. Hitherto, 41 RTHa cases have been recorded worldwide. Methods: RTHa cases (n = 10) attending a single center underwent cutaneous assessment, recording skin lesions. Lesions excised from different RTHa patients were analyzed histologically and profiled for cellular markers of proliferation and oncogenic potential. Proliferative characteristics of dermal fibroblasts and inducible pluripotent stem cell (iPSC)-derived keratinocytes from patients and control subjects were analyzed. Results: Multiple skin tags and nevi were recorded in all cases, mainly in the head and neck area with a predilection for flexures. The affected patients had highly deleterious mutations (p.E403X, p.E403K, p.F397fs406X, p.A382PfsX7) involving TRa1 alone or mild/moderate loss-of-function mutations (p.A263V, p.L274P) common to TRa1 and TRa2 isoforms. In four patients, although lesions excised for cosmetic reasons were benign intradermal melanocytic nevi histologically, they significantly overexpressed markers of cell proliferation (K17, cyclin D1) and type 3 deiodinase. In addition, oncogenic markers typical of basal cell carcinoma (Gli-1, Gli-2, Ptch-1, n = 2 cases) and melanoma (c-kit, MAGE, CDK4, n = 1) were markedly upregulated in skin lesions. Cell cycle progression and proliferation of TRa mutation-containing dermal fibroblasts and iPSC-derived keratinocytes from patients were markedly increased. Conclusions: Our observations highlight frequent occurrence of skin tags and benign melanocytic nevi in RTHa, with cutaneous cells from patients being in a hyperproliferative state. Such excess of skin lesions, including nevi expressing oncogenic markers, indicates that dermatologic surveillance of RTHa patients, monitoring lesions for features that are suspicious for neoplastic change, is warranted.