Hypothyroidism is one of the most common hormonal disorders in the general population and, in most cases, its treatment is simple and highly effective. However, when it occurs in people with cancer, the situation changes.
In this context, hypothyroidism often goes unnoticed, not because it is rare, but because its symptoms are easily confused with those of cancer itself or with the side effects of oncologic treatments.
Persistent fatigue, weight gain, difficulty concentrating, depression, cold intolerance, or constipation are common symptoms of hypothyroidism, but they are also extremely common in patients with cancer.
This overlap often leads to delayed recognition of thyroid dysfunction, postponing a treatment that could significantly improve the patient’s quality of life.
For many years, hypothyroidism was considered a condition typical of older women, mainly associated with autoimmune thyroid diseases. However, in recent decades, an important shift has been observed: increasing numbers of younger individuals and men are developing hypothyroidism as an indirect consequence of cancer treatment.
This phenomenon is closely related to advances in oncology. Radiotherapy, radioiodine therapy, and new targeted therapies have improved survival for many patients, but they have also revealed late side effects that were previously less evident. Among these, thyroid dysfunction occupies a prominent place.
We now know that patients treated for head and neck tumors, lymphomas, breast cancer, brain tumors, or renal cancer have a considerable risk of developing hypothyroidism, even several years after completing oncologic treatment.
The thyroid is a small but highly sensitive gland. Radiotherapy applied to areas near the neck can damage it progressively. This damage is not always immediate; in many cases, the gland deteriorates slowly over the years until it can no longer produce sufficient hormones.
Radiation can affect both the cells that produce thyroid hormones and the small blood vessels that supply them. Over time, this leads to inflammation, scarring, and loss of function. As a result, hypothyroidism may appear even two to seven years after treatment.
In addition, some treatments with radioactive iodine, used both in thyroid disease and in certain oncologic contexts, directly destroy thyroid cells. In these cases, hypothyroidism is usually permanent and requires lifelong treatment.
Various medications used in modern oncology further contribute to this problem. Some targeted therapies and immunologic drugs can interfere with thyroid hormone production or accelerate their clearance from the body. Interestingly, in certain types of cancer, the development of hypothyroidism during these treatments has been associated with a better tumor response, reflecting the complex relationship between the immune system, hormones, and cancer.
In some patients, hypothyroidism does not originate in the thyroid gland itself, but in the brain. This occurs when the hypothalamus or pituitary gland—the structures that regulate thyroid function—are affected by tumors, surgery, or radiotherapy.
In these cases, the thyroid gland is structurally normal but does not receive the proper signal to function. This type of hypothyroidism is more difficult to detect, because routine laboratory tests may appear normal if they are not carefully interpreted. For this reason, in patients with brain cancer or those treated with cranial radiotherapy, a more comprehensive hormonal evaluation is essential.
Although extreme fatigue is the most common symptom, hypothyroidism can affect nearly every organ system. At the cardiovascular level, it can slow the heart rate and reduce the heart’s ability to pump blood efficiently. At the mental level, it can cause memory problems, difficulty concentrating, depression, and even more severe mental disturbances in advanced cases.
In patients with cancer, these effects can be particularly harmful. Untreated hypothyroidism may reduce tolerance to chemotherapy, worsen overall well-being, and interfere with recovery between treatment cycles. In children and adolescents, the consequences are even more significant, as growth and neurological development may be affected.
From a technical standpoint, detecting hypothyroidism is relatively simple. A blood test measuring thyroid-stimulating hormone (TSH) is usually sufficient to identify most cases. However, in patients with cancer, ordering the test is not enough; clinicians must actively consider the diagnosis.
When persistent symptoms cannot be fully explained by cancer or its treatment, evaluating thyroid function can make a substantial difference. In some cases, additional thyroid hormone measurements are required to obtain a more complete picture.
Figure 1. Thyroid dysfunction represents a frequent but often underrecognized consequence of cancer and its treatment. Because its clinical manifestations overlap with those of malignancy and therapy-related toxicity, active hormonal monitoring is essential to identify hypothyroidism early and optimize long-term patient care.
The treatment of hypothyroidism consists of replacing the hormone that the body no longer produces in adequate amounts. This therapy is safe, inexpensive, and highly effective. Nevertheless, in patients with cancer, the appropriate dose may change over time.
Weight changes, digestive alterations, interactions with other medications, and the patient’s overall condition can all modify hormone requirements. Therefore, regular follow-up and individualized dose adjustments are essential to avoid both hormone deficiency and excess.
Hypothyroidism is not a minor issue in the context of cancer. It is a frequent and often silent condition, but one with a profound impact on quality of life and response to oncologic treatments.
Routinely incorporating thyroid evaluation into the follow-up of patients with cancer—especially those receiving radiotherapy or targeted therapies—not only allows for the detection of a treatable condition, but also strengthens a more comprehensive approach to oncologic care.
Recognizing and treating hypothyroidism in a timely manner is a concrete way to humanize cancer medicine, placing the patient at the center rather than focusing solely on the disease.
Main reference:
Carter, Y., Sippel, R. S., & Chen, H. (2014). Hypothyroidism after a cancer diagnosis: etiology, diagnosis, complications, and management. The oncologist, 19(1), 34–43. https://doi.org/10.1634/theoncologist.2013-0237
Other reference:
Roberts, C. G., & Ladenson, P. W. (2004). Hypothyroidism. Lancet (London, England), 363(9411), 793–803. https://doi.org/10.1016/S0140-6736(04)15696-1
ROITT, I. M., DONIACH, D., CAMPBELL, P. N., & HUDSON, R. V. (1956). Auto-antibodies in Hashimoto’s disease (lymphadenoid goitre). Lancet (London, England), 271(6947), 820–821. https://doi.org/10.1016/s0140-6736(56)92249-8
Harington, C. R., & Barger, G. (1927). Chemistry of Thyroxine: Constitution and Synthesis of Thyroxine. The Biochemical journal, 21(1), 169–183. https://doi.org/10.1042/bj0210169
