Primary hypothyroidism is most common – most likely an autoimmune process as a result of Hashimoto’s thyroiditis. If severe in adults  myxoedema; in infancy  cretinism.
Secondary – deficient TRH from hypothalamus or TSH from pituitary; post-thyroidectomy or after radioiodine treatment

Clinical features:
Tiredness, dull facial expression (“unhappy”), hoarse voice, slow speech, facial puffiness and appears pale, cold intolerance, hair is sparse, dry, scaly, cold and pale skin, some weight gain, gradual change to personality, delayed/slowed relaxation of reflexes, anaemia, caratemaemia of palms and soles (if severe and longstanding), polyarthritis (insidious onset of pain and stiffness and maybe swelling – most commonly knees – synovial thickening ligamentous laxity and effusions are characteristic). 7-10% can have carpel tunnel syndrome. Avascular necrosis of hip is more common. Raynaud’s phenomenon can occur. Aching muscles similar to fibromyalgia (differential may be initially confusing). Often difficult to diagnoses in the elderly, as many of the features of hypothyroidism can appear to be part of the natural ageing process.
Skin is thicker due to increases dermal mucopolysaccharide (myxoedema) – reversible with thyroid replacement.
Diagnosis – clinical features and laboratory evaluation; early diagnosis is uncommon – usually diagnosed at advanced stages or incidentally diagnosed.

Hormone replacement therapy – thyroxine (T4).

Involvement of feet:
Skin can be dry, coarse and scaly – heel fissures common.
Carotenaemia of plantar surface – due to deposition of carotene in lipid rich epidermal layers.
Metatarsophalangeal joints can be affected – insidious onset of pain and stiffness – often worse in morning (symptoms mimic rheumatoid arthritis).
Depressed deep tendon reflexes with slow relaxation phase
Peripheral neuropathies (eg tarsal tunnel syndrome) may develop – also can be a distal sensorimotor or sensory distal polyneuropathy – starts as paraesthesia and pain; touch, vibration and proprioception are decreased; the polyneuropathy most commonly start as symmetric numbness in feet

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