Hypothyroidism

Summary

Hypothyroidism is a condition characterized by a deficiency of thyroid hormones . Iodine deficiency is the most common cause worldwide, but Hashimoto’s disease (autoimmune thyroiditis ) is the leading cause in non–iodine-deficient regions. Primary hypothyroidism occurs with thyroid gland disorders, while the central type arises from pituitary and hypothalamic conditions. Thyroid hormones are integral in metabolic processes and in the development of the brain and other organs. Congenital hypothyroidism can result in significant mental disability due to the loss of thyroid hormones . The features of acquired hypothyroidism also reflect the effects of slowed organ function, such as fatigue, bradycardia, cold intolerance, and exertional dyspnea . Diagnosis is by thyroid function testing. Elevated thyroid stimulating hormone and low free thyroxine (T4) levels are noted. Treatment is with synthetic T4.

Classification

• Primary hypothyroidism (thyroid gland involved) ➜ High TSH
  • Low thyroxine/tetraiodothyronine (T4)
  • Low triiodothyronine (T3)
• Secondary (pituitary involved): low thyroid stimulating hormone (TSH)
• Tertiary (hypothalamus involved): low thyrotropin-releasing hormone (TRH)

• Primary hypothyroidism: insufficient thyroid hormone production
  • Hashimoto thyroiditis
    • The most common cause of hypothyroidism in iodine-sufficient regions
    • Associated with other autoimmune diseases (e.g., vitiligo, pernicious anemia, type 1 diabetes mellitus, and systemic lupus erythematosus)
  • Postpartum thyroiditis (subacute lymphocytic thyroiditis)
  • De Quervain thyroiditis (subacute granulomatous thyroiditis): often subsequent to a flu-like illness
  • Iatrogenic: e.g., post thyroidectomy, radioiodine therapy, antithyroid medication (e.g., amiodarone, lithium)
  • Nutritional (insufficient intake of iodine): the most common cause of hypothyroidism worldwide, particularly in iodine-deficient regions
• Secondary hypothyroidism: pituitary disorders (e.g., pituitary adenoma) ➜ TSH deficiency ➜ T4 & T3 deficiency
• Tertiary hypothyroidism: hypothalamic disorders à TRH deficiency ➜ TSH deficiency ➜ T4 & T3 deficiency
  • Increased TRH levels will stimulate the release of prolactin

Clinical features

• Symptoms related to decreased metabolic rate
  • Fatigue, decreased physical activity
  • Cold intolerance
  • Decreased sweating
  • Hair loss, brittle nails, and cold, dry skin
  • Weight gain (despite poor appetite)
  • Constipation
  • Bradycardia
  • Hypothyroid myopathy, myalgia, stiffness, cramps
  • Woltman sign: a delayed relaxation of the deep tendon reflexes, which is commonly seen in patients with hypothyroidism, but can also be associated with advanced age, pregnancy, and diabetes mellitus.
  • Entrapment syndromes (e.g., carpal tunnel syndrome)
• Symptoms related to generalized myxedema
  • Doughy skin texture, puffy appearance
  • Myxedematous heart disease (dilated cardiomyopathy, bradycardia, dyspnea)
  • Hoarse voice, difficulty articulating words
  • Pretibial and periorbital edema
  • Myxedema coma
• Symptoms of hyperprolactinemia
  • Abnormal menstrual cycle (esp. secondary amenorrhea; or menorrhagia)
  • Galactorrhea
  • Decreased libido, erectile dysfunction, delayed ejaculation, and infertility in men

Diagnosis

Further tests could be used to determine the exact cause of hypothyroidism

• Antibody testing ➜ Thyroglobulin and thyroid peroxidase antibodies (TPOs):
  • Consistent with autoimmune hypothyroidism
• Imaging studies:
  • Ultrasonography: rarely used but can help in distinguishing asymmetric goiter in Hashimoto’s thyroiditis from a nodule/multinodular goiter
  • Brain imaging:
    • CT or MRI
    • Part of workup for central hypothyroidism (secondary and tertiary hypothyroidism)
    • Assess pituitary gland, hypothalamus, and surrounding areas.

Treatment

• L-thyroxine replacement
• L-thyroxine replacement in subclinical hypothyroidism if:
  • TSH ≥ 10 mU/L
  • TSH 7.0–9.9 mU/L in asymptomatic patients < 70 years
  • TSH above the upper limit of normal to 6.9 mU/L in symptomatic patients < 70 years
• For central hypothyroidism:
  • Thyroid replacement
  • Treat other concomitant hormone deficiencies, if present.

Complications

• Myxedema coma
• Primary thyroid lymphoma (Hashimoto thyroiditis is the most common cause of hypothyroidism and the only known risk factor for primary thyroid lymphoma)
• Carpal tunnel syndrome
• Increased cardiovascular risk (Hypothyroidism is associated with hypercholesterolemia, which in turn increases cardiovascular risk.)

<span id="whg4RgYWx4">Hashimoto Thyroiditis</span>

• It is an autoimmune disorder leading to destruction of the thyroid cells and thyroid failure leading to hypothyroidism.
• The gradual clinical course of Hashimoto’s thyroiditis starts with a transient hyperthyroid state (“hashitoxicosis”) followed by subclinical hypothyroidism then progression to overt hypothyroidism occurs, which is permanent.
• Hashimoto’s thyroiditis, or chronic lymphocytic thyroiditis, is the most common cause of hypothyroidism in iodine-sufficient regions
• More common in women than men: 10–20:1 ratio

Other Thyroiditis

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