Hypothyroidism Diagnosis

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

• 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

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

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

• 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

• 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