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 TSH and low FT4
- Subclinical hypothyroidism: High TSH and normal FT4
Other Tests
- Lipid profile
- 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
- Temporary increase in TSH due to calorie intake restriction
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
- Thyroid ultrasound
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
- 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