hypothyroidism
HYPOTHYROIDISM
Hypothyroidism is a common endocrine disorder where in the thyroid does not make enough thyroid hormone.
Subclinical/mild hypothyroidism refers to the state of slightly increased serum TSH with normal serum FT4 in patients who are usually asymptomatic.
Most common cause of primary hypothyroidism is autoimmune thyroiditis or Hashimoto's disease.
Levothyroxine is the first-line agent for treatment of hypothyroidism.

Hypothyroidism Diagnosis

Laboratory Tests

Confirmation & Types of Hypothyroidism

  • Measure serum thyroid stimulating hormone (TSH): Most sensitive test for detecting hypothyroidism
  • Confirm with measurement of serum thyroxine (FT4)
  • Primary hypothyroidism: High TSH and low FT4
  • Central hypothyroidism: Low/normal/mildly elevated TSH and low FT4
  • Subclinical hypothyroidism: High TSH and normal FT4

Other Tests

  • Thyroid autoantibodies
    • Antithyroid peroxidase, antithyroglobulin autoantibodies
  • Thyroid scan, ultrasonography (may do both)
  • Neuroradiologic studies [magnetic resonance imaging (MRI) or computed tomography (CT)]
    • Recommended in patients with biochemical evidence of central hypothyroidism to assess the hypothalamic-pituitary region

Subclinical / Mild Hypothyroidism

  • Refers to the state of slightly increased serum TSH with normal serum FT4 in patients who are usually asymptomatic

Other Causes of Elevated TSH that Should be Excluded

  • Recent Levothyroxine dose adjustment wherein a steady state is not achieved
  • Transient increase in TSH during recovery from severe illness or destructive thyroiditis
  • Untreated primary adrenal insufficiency
  • Administration of recombinant human TSH
  • Presence of heterophilic antibodies against mouse proteins

Determination of Treatment for Subclinical Hypothyroidism:

Further Evaluation

  • Thyroid antibodies detection (if positive, for treatment); if negative (TSH >10 mU/L, for treatment)
  • Physical exam
  • Lipid profile

Risk Assessment

  • Higher risk of progression to overt hypothyroidism if TSH >10 mU/L, presence of thyroid antibodies, goiter, hyperlipidemia, pregnancy, ovulatory dysfunction with infertility
    • Treatment is recommended in higher risk patients
    • If patient is not at high risk, perform annual thyroid exam

Primary Hypothyroidism

Etiologies

Autoimmune Thyroiditis (Hashimoto’s Disease)

  • Most common cause
  • Diagnosis
    • Thyroid antibodies [anti-thyroid peroxidase (TPO)- in 95% or antithyroglobulin (TG) detection in 60%]
    • Careful history of radiation exposure, radioactive iodine therapy, neck surgery, recent viral infection, medications, pregnancy, change in diet
    • Presence of thyroid antibodies and absence of any of the above history confirms diagnosis of Hashimoto’s thyroiditis

Other Causes

  • Congenital: Endemic iodine deficiency (Athyreosis, dyshormonogenesis), thyroid hormone resistance, TSH-receptor defect
  • Acquired: Iodine deficiency, iatrogenic (post-thyroidectomy, thyroid irradiation, medications, radioactive iodine therapy)
  • Transient: Subacute thyroiditis (de Quervain’s), lymphocytic thyroiditis (silent, postpartum and painless thyroiditis), neonatal hypothyroidism

Central Hypothyroidism

Evaluation

  • Thorough exam of the function of the hypothalamic-pituitary axis
  • Determine adrenal status, if adrenal insufficient:
    • Thyroxine administration may increase metabolism and precipitate an adrenal crisis
    • Glucocorticoid should be replaced prior to thyroxine administration to avoid adrenal crisis

Etiologies 

  • Divided into secondary hypothyroidism (defect is in the pituitary gland) and tertiary hypothyroidism (defect is in the hypothalamus)
  • Occurs due to failure of the hypothalamic-pituitary axis causing decreased TSH secretion or reduced TSH biological activity
  • Pituitary: Tumor, vascular insufficiency, empty sella syndrome, infiltrating disease, infection, iatrogenic (surgery and radiotherapy)
  • Hypothalamic: Tumor, infection, vascular insufficiency, thyrotropin-releasing hormone (TRH) deficiency, iatrogenic

Myxedema Coma

  • Correct diagnosis is imperative because critical illnesses are clinically similar to myxedema coma and can also present with altered thyroid function
    • If diagnosis is wrong, high doses of thyroid hormone is dangerous to the patient
  • A high suspicion of myxedema coma can be made in a poorly responsive patient with a thyroidectomy scar or history of radioiodine therapy or hypothyroidism
    • There may also be antecedent symptoms of thyroid dysfunction followed by progressive lethargy, stupor and coma

Signs and Symptoms

  • Severe hypothermia (<27°C)
  • Bradycardia
  • Respiratory failure and loss of consciousness
  • Hypercapnia
  • Hyponatremia
  • Long-standing hypothyroidism

Precipitating Factors

  • Infection
  • Cardiovascular event
  • History of thyroidectomy or radioiodine therapy
  • Exposure to cold
Diagnostic Tests
  • Very low serum T4 concentration
  • High serum TSH concentration
  • Diagnostic scoring system for myxedema coma gives points for the degree of hypothermia, neurological effects, gastrointestinal findings, precipitating event, cardiovascular dysfunction, and metabolic disturbances
    • Score of ≥60 is highly suggestive of myxedema coma, while scores between 45 & 59 classify the patient at risk for myxedema coma
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