How common is hypothyroidism in postradiation head and neck cancer patients?

The incidence of postradiation hypothyroidism is hard to determine, as the literature is quite varied. This variation is due to a combination of how hypothyroidism is measured, how much of the thyroid was treated, and when patients were assessed. Recent literature suggests the incidence can approximate 50%, and an often quoted paper is one from Cleveland where the 5 yr rate was 48%; interestingly the 8-yr rate was 67%.

There is agreement that clinical hypothyroidism should be treated, but often this is a challenging diagnosis. Symptoms include fatigue, constipation and cold intolerance, all which mimic post XRT symptoms in the first year, and even weight gain is not a clincher, since ideally as XRT induced odonyphagia resolves, pts will gain weight.

I think there is also relative agreement in patients that are without symptoms, but have TSH > 10, they should be treated.

The high normal value for TSH is ~ 4-5, and there tends to be controversy about the role of thyroid supplement/replacement in pts with TSH 5 - 10. Some of my colleagues will automatically start patients on Synthroid (or equivalent) with any elevated TSH, arguing that it will ultimately get worse over time. As TSH seems much less sensitive than say PSA where we look at decimal values, I often like to repeat it at next visit to see if it was correct when minimally elevated. I tend to treat chemical hypothyroidism if I see a trend toward continuing to rise in the TSH, or the minimally elevated TSH is accompanied with a low or low normal T4.

Radiation induced hypothyroidism (RIHT) is common. As described by @Dr. First Last and @Dr. First Last, the incidence varies in literature. We analyzed 117 pts at VCU, with median Fu of 3 years, 32% developed RIHT (> 5.5), median time is 18 months after RT; nearly 1/3 were detected in the 1st year.

At our institution, we draw TSH every 3-6 months in the first 2 years, then every 6-12 months or in pt c/o fatigue or other symptoms. Both Rad Onc and ENT manage RIHT, whoever the first to make the diagnosis. If TSH is slightly elevated and without symptoms, we may observe. In many cases, after initial prescription, we transfer RIHT management to PCP.

Incidence of radiation induced hypothyroidism after radiation to the cervical neck has been reported to be around 20 to 30%.  Most patients develop hypothyroidism within the first 5 years of follow-up.   Although the exact dose response remains elusive, most studies indicate that the risk of hypothyroidism increases with high dose to the thyroid gland. 

I don’t usually prescribe levothyroxine for a patient in the euthyroid state until all other attempts have failed to alleviate detrimental symptoms.