Investigation of Thyroid Dysfunction

Adapted from the Thyroid Testing Guidelines for Alberta Physicians - April 1992.

Thyroid stimulating hormone (TSH) is the initial test for the diagnosis of primary hyperthyroidism and hypothyroidism.  The measurement of TSH levels is the most sensitive and specific test for thyroid dysfunction.  The new generation of sensitive immunoradiometric TSH assays can reliably detect a concentration as low as 0.01 mU/L.  Reference values range between 0.15 mU/L and 6 mU/L, but some variations are possible in different laboratories.  Sensitivity and specificity of TSH is in excess of 90 percent in detecting thyroid disease (Caldwell, 1985).

Exclusions

• TSH is not reliable in cases of suspected pituitary disease - FT4 is recommended in this circumstance.
• TSH may be an unreliable indicator of thyroid status in patients with acute severe nonthyroidal illness (e.g., CCU, ICU, acute severe psychiatric illness).
• Within 3 months of a change in therapy for hyperthyroidism TSH may not reflect the clinical status.  During this time FT4 or FT3 is recommended.

Patients at Increased Risk for Thyroid Disease

• Women over 45
• Postpartum women
• Patients receiving drug therapies such as lithium and amiodarone
• Patients with other autoimmune diseases such as Type I diabetes

Recommendations

• When patients are asymptomatic, seemingly healthy or having a periodic examination: no testing is required.
• When patients have non-specific symptoms of thyroid disease, but do not belong to a group at increased risk for thyroid disease: testing is not recommended.
• When patients have non-specific symptoms of thyroid disease and are in a group at increased risk for thyroid disease:  measure TSH and follow Category 1.
• When patients have definite clinical signs of thyroid disease:  follow Category 1.
• When patients are taking thyroid hormone replacement and dosage needs monitoring:  follow Category 2.
• When patients are receiving thyroxine therapy for goitre and thyroid tumors:  follow Category 3.

The above recommendations are systematically developed statements to assist practitioner decisions about appropriate laboratory testing for specific medical conditions.

Background

The Canadian Task Force on the Periodic Health Examination (1994) have reported that the prevalence rates of hyperthyroidism are 1.9 to 2.7 percent. The U.S. Preventive Services Task Force (1989) estimate that between 2 and 3 percent of the U.S. population have either hypothyroidism or hyperthyroidism.  The annual incidence rate of overt hyperthyroidism has been estimated to be two to three per 1,000 women.  The Thyroid Foundation of Canada estimates that thyroid disorders affect one in twenty Canadians and that most thyroid disorders are 5 - 7 times more common in women.

The prevalence of hypothyroidism is three times higher among women than men.  The prevalence in an unselected community population of young, middle aged and elderly individuals is about 1.4 percent and the estimated annual incidence rate is one to two per 1,000 women.  Surveys of geriatric populations have yielded estimated prevalence rates for overt hypothyroidism of 0.2 percent to 3 percent (Canadian Task Force on the Periodic Health Examination, 1994).  The presentation of symptoms in the elderly may be atypical or absent (Singer et al, 1995; U.S. Preventive Services Task Force, 1989).  The reported prevalence of subclinical hypothyroidism ranges from 0.9 percent to 5.2 percent in the adult population and from 2.6 percent to 20 percent in the elderly population.

Symptoms of Hypothyroidism

• Weight gain
• Lethargy
• Cold intolerance
• Menstrual irregularities
• Depression
• Constipation
• Dry Skin

Symptoms of Hyperthyroidism

• Palpitations/Tachycardia
• Widened pulse pressure
• Nervousness and tremor
• Heat intolerance
• Weight loss
• Muscular weakness
• Goitre is usually present

Category 1 – Suspected Hyper or Hypothyroidism

Patients with thyrotoxicosis usually have TSH values less than 0.1 mU/L.

• TSH values between 0.1 and 0.2 mU/L are rarely seen in primary thyrotoxic patients and usually are the result of excess thyroxine therapy.  If not undergoing thyroxine treatment, repeat test in 1 month before proceeding to consultation.
• Thyroid antibodies are indicated in cases of hypothyroidism (TSH 6 - 12 mU/L) due to suspected autoimmune thyroid disease.  Serum antibody testing should only be performed for diagnosis.

Category 2 - TSH Use In Thyroxine Therapy For Treatment of Hypothyroidism

• Target:  TSH in normal range.
• Thyroxine doses should be adjusted no more frequently than 8 - 10 week intervals.
• Once a stable dose is achieved, yearly TSH is sufficient.

Category 3 - TSH Use In Monitoring Thyroxine Therapy In Goitre and Thyroid Tumours

Thyroid Cancer

• Target: Achieve suppressed TSH (< 0.2 mU/L) to prevent regrowth of tumours.

Benign Goitre or Nodules

• Target:  TSH:  0.1 - 1.0 mU/L.