Thyroid Cancer and Thyroid Function Status
Thyroid cancer itself is neither hyperthyroidism nor hypothyroidism, but most commonly presents as euthyroid (normal thyroid function). 1 However, thyroid function can be affected depending on the type, stage, and extent of the cancer.
Typical Thyroid Function in Thyroid Cancer
- Most thyroid cancer patients have normal thyroid function at diagnosis
- Thyroid function tests (TSH, Free T4) are typically within normal ranges
- Thyroid cancer does not inherently cause dysfunction of hormone production in most cases
Exceptions: When Thyroid Cancer Can Affect Thyroid Function
Rare Cases of Hyperthyroidism in Thyroid Cancer
- Hyperfunctioning thyroid cancer (causing hyperthyroidism) is rare but documented 2
- Occurs primarily in:
- Large metastatic follicular thyroid carcinomas with significant tumor burden
- Cases where the large tumor mass produces excessive thyroid hormone despite poor efficiency
- T3 toxicosis (elevated T3 with normal T4) is common in these cases
Hypothyroidism in Thyroid Cancer
- Hypothyroidism is not typically caused by the cancer itself
- Becomes common after treatment:
- Following total thyroidectomy (surgical removal of the thyroid)
- After radioactive iodine (RAI) therapy
- These treatments necessitate lifelong thyroid hormone replacement 1
Thyroid Function Disorders in Cancer Patients
- Cancer patients (even non-thyroid cancers) have higher rates of thyroid dysfunction (29.5%) compared to healthy individuals (15.4%) 3
- Euthyroid sick syndrome is the most common thyroid abnormality in cancer patients (14%) 3
- Both hyperthyroidism and hypothyroidism are more prevalent in cancer patients than in the general population
Anaplastic Thyroid Cancer Considerations
- Anaplastic thyroid carcinoma (ATC) is an aggressive form with poor prognosis
- Median survival from diagnosis is approximately 5 months 1
- Thyroid function status is not typically affected by the cancer itself, but by treatment
- Patients require levothyroxine if total thyroidectomy is performed 1
Clinical Implications
- Thyroid function tests (TSH, Free T4) should be part of the initial workup for thyroid cancer
- Imaging studies like ultrasound are used to evaluate thyroid morphology but do not help differentiate causes of thyroid dysfunction 1
- After thyroidectomy for thyroid cancer, patients will require lifelong thyroid hormone replacement to prevent hypothyroidism
- Higher thyroid hormone levels have been associated with advanced clinical stages of certain cancers 4, 5
Common Pitfalls
- Failing to monitor thyroid function before, during, and after thyroid cancer treatment
- Overlooking the possibility of hyperthyroidism in metastatic follicular carcinoma
- Not recognizing that T3 toxicosis may occur with normal T4 levels in functioning thyroid cancer 2
- Assuming all thyroid nodules found on imaging represent thyroid dysfunction (they often do not)
In summary, while thyroid cancer itself typically does not cause hyper- or hypothyroidism at presentation, thyroid function should be carefully monitored throughout the disease course and treatment, as dysfunction can develop due to treatment effects or, rarely, from the tumor itself.