From the Guidelines
A low TSH workup should begin with checking TSH and FT4 levels to determine the cause of the low TSH, and consideration of TSH receptor antibody testing if clinical features suggest Graves’ disease. According to the ASCO guideline update 1, TSH can be checked every 4-6 weeks as part of routine clinical monitoring for asymptomatic patients on ICPi therapy. For symptomatic patients, TSH and FT4 should be used for case detection, and T3 can be helpful in highly symptomatic patients with minimal FT4 elevations.
Key Considerations
- Low TSH with a low FT4 is consistent with central hypothyroidism, and evaluation should be done as per hypophysitis.
- Clinical features and suspicion of Graves’ disease (e.g., ophthalmopathy and T3 toxicosis) should prompt consideration of TSH receptor antibody testing.
Management
- For asymptomatic or mild symptoms (G1), patients can continue ICPi, and beta-blocker (e.g., atenolol or propranolol) can be used for symptomatic relief.
- For moderate symptoms (G2), consideration of holding ICPi and endocrine consultation is recommended.
- For severe symptoms (G3-4), ICPi should be held, and endocrine consultation and hospitalization may be necessary. It is essential to monitor thyroid function every 2-3 weeks after diagnosis to catch the transition to hypothyroidism, which is the most common outcome for transient subacute thyroiditis, as noted in the guideline update 1.
From the Research
Low TSH Workup
- A low TSH level can indicate hyperthyroidism, which is an excessive concentration of thyroid hormones in tissues caused by increased synthesis of thyroid hormones, excessive release of preformed thyroid hormones, or an endogenous or exogenous extrathyroidal source 2.
- The most common causes of an excessive production of thyroid hormones are Graves disease, toxic multinodular goiter, and toxic adenoma 2, 3.
- Hyperthyroidism can be treated with antithyroid medications, radioactive iodine ablation of the thyroid gland, or surgical thyroidectomy 2, 3, 4.
- The choice of treatment depends on the underlying diagnosis, the presence of contraindications to a particular treatment modality, the severity of hyperthyroidism, and the patient's preference 2, 3.
Diagnostic Approach
- The diagnosis of hyperthyroidism can be made based on clinical findings and confirmed with biochemical tests and imaging techniques including ultrasound and radioactive iodine uptake scans 4.
- Thyroid function tests, including TSH and free thyroxine (FT4) levels, can help establish the diagnosis of hyperthyroidism 3, 4.
- Thyroid scintigraphy is recommended if thyroid nodules are present or the etiology is unclear 3.
Treatment Options
- Treatment options for overt hyperthyroidism from autonomous thyroid nodules or Graves disease include antithyroid drugs, radioactive iodine ablation, and surgery 3, 4.
- Treatment for subclinical hyperthyroidism is recommended for patients at highest risk of osteoporosis and cardiovascular disease, such as those older than 65 years or with persistent serum thyrotropin level less than 0.1 mIU/L 3.
- Beta blockers can be used to treat cardiovascular manifestations of hyperthyroidism, such as hypertension and tachycardia 4.