Thyroid disorders are characterized by abnormal levels of thyroid hormone in the serum. Thyroid hormone has many roles in body metabolism. These include its involvement in the embryonic development of mammalian skin, the initiation and maintenance of hair growth, determination of epidermal thickness, and secretion of sebum.

When the normal amount of thyroid hormone is present to, the person is referred to as euthyroid. When the amount is excessive or reduced from normal, as may occur due to overactivity and underactivity of the thyroid gland, it may result in cutaneous changes, of various types. These skin alterations depend on the type of thyroid disease that occurs.

Hypothyroidism

Primary hypothyroidism most commonly occurs as a result of autoimmune disease resulting in thyroid glandular failure.  The effect may be due to either inadequate circulating levels of thyroid hormone, or the resistance of target cells to thyroid action. The symptoms include those below:

• Myxedema: this is caused by an elevated level of mucopolysaccharides (chondroitin sulfate and hyaluronic acid). These accumulate in the dermis, most prominently around the hair follicles and dermal vessels. In addition to separating the collagen bundles, they may produce some collagen degeneration.
• Pale, thin, and wrinkled skin: the paleness is due to the abnormal dermal content of mucopolysaccharide and water.
• Cold peripheries: a reduced core temperature and diminished skin perfusion occur due to cutaneous vasoconstriction.
• Extremely dry skin: this can result from decreased eccrine gland secretion. In very severe cases sweating processes may be disrupted. This results in the palms and soles becoming thick and dry, a condition known as keratoderma, or splitting of the skin surface with a characteristic paving-stone appearance termed eczema craquelé.
• Carotenemia:  this refers to a yellowish discoloration on the palms and soles caused by an increased level of dermal carotene.
• Slow growing dry, brittle, coarse hair and diminished body hair: the drying and coarseness are considered to be partly due to the reduced secretion of sebum.
• Madarosis: this refers to the loss of the lateral third of the eyebrows.
• The presence of coarse, thin, and brittle nails which grow slowly
• Poor wound healing tendency: healing becomes slow in proportion to the degree of thyroid hormone deficiency.
• Hypohidrosis: this refers to an abnormal lack of sweating.
• Xerosis: in 57-59% of patients there is a change in skin texture and poor skin hydration. This results in the skin becoming rough and covered in fine scales.

Hyperthyroidism  

The precise pathophysiology that underlies the characteristic cutaneous symptoms of hyperthyroidism still requires elucidation. Some associated symptoms are as follows:

• Smooth, soft, thin, warm, and moist skin: the epidermis is not atrophic but is thin, and the outermost skin layer is adequately hydrated. An increase in cutaneous blood flow causes the warmth which often occurs along with redness of the face, elbows, and palms (palmar erythema).
• Fine, soft hair: this might occur with a diffuse non-scarring form of hair loss (alopecia).
• Plummer's nails: this is characterized by a concave contouring which raises the nails away from the nail bed. It is accompanied by distal onycholysis with hyperpigmentation.
• Telangiectasia: this pattern of small visible capillary vessels in a linear fashion results from breakage of capillaries.
• Hyperhydrosis: this abnormally excessive sweating may be observed particularly on the palms and soles.
• Pruritus pretibial myxedema: this is a mucinosis in which excessive amounts of mucopolysaccharides accumulate in the dermis and subcutis of the skin, to cause localized thickening of the pretibial skin. It is nearly always associated with Graves’ disease. Incidentally, hyperthyroidism affects a small number of patients with Graves’ disease (<5%) and commonly occurs within 2 years after the diagnosis.
• Tender, red-brown nodules occur on areas such as the shins, calves and feet.
• The development of early graying of hair, coupled with a decrease in pigment content of hair, may present as an early symptom.
• Both generalized and localized hyperpigmentation may occur: this might be due to the elevated release of adrenocorticotropic hormone (ACTH) from the pituitary, to compensate for the upregulated degradation of cortisol.
• Thyroid acropachy: this extremely uncommon and unusual condition consists of both digital clubbing and swelling in addition to periosteal reactions.

References

• http://www.thyroid.ca/e9b.php
• http://austinpublishinggroup.com/thyroid-research/fulltext/thyroids-v1-id1008.php
• http://www.dermnetnz.org/dermal-infiltrative/pretibial-myxoedema.html
• http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3371544/
• http://www.allthyroid.org/disorders/hyperthyroidism/overactive.html
• http://www.dermnetnz.org/vascular/telangiectasia.html
• http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4138926/
• http://www.dermnetnz.org/dermatitis/eczema-craquele.html
• http://www.dermnetnz.org/vascular/palmar-erythema.html