Which of the following combinations of free thyroxine (FT4) level and thyroid-stimulating hormone (TSH) level is MOST likely in a patient with subclinical hypothyroidism?

  • □ (A) Low FT4, low TSH
  • □ (B) Normal FT4, high TSH
  • □ (C) High FT4, low TSH

About 5% of patients older than 65 years have hypothyroidism. Primary hypothyroidism is due to thyroid gland dysfunction, while secondary hypothyroidism is due to pituitary dysfunction. In primary hypothyroidism, thyroid-stimulating hormone (TSH) is increased due to a negative feedback loop that stimulates the failing gland, while free thyroxine (FT4) is low or low-normal. Low levels of both FT4 and TSH suggest secondary hypothyroidism, often associated with pituitary dysfunction. High FT4 levels with low TSH levels suggest primary hyperthyroidism (Figure).

 

Figure: The thyroid gland is controlled by the pituitary in a feedback loop. In the normal thyroid (left), increasing amounts of FT4 leads to a decrease in TSH secretion by the pituitary. Several abnormal states are depicted in subsequent panels. The bold arrows indicate increased amounts; the dotted arrows indicate decreased amounts. © 2021 American Society of Anesthesiologists.

Figure: The thyroid gland is controlled by the pituitary in a feedback loop. In the normal thyroid (left), increasing amounts of FT4 leads to a decrease in TSH secretion by the pituitary. Several abnormal states are depicted in subsequent panels. The bold arrows indicate increased amounts; the dotted arrows indicate decreased amounts. © 2021 American Society of Anesthesiologists.

Subclinical hypothyroidism is characterized by normal FT4 and high TSH. High TSH ranges from 5 to 15 mU/L, while normal TSH ranges from 0.3 to 4.5 mU/L. Generally, subclinical hypothyroidism is thought to have little or no perioperative significance. An increasing TSH level is the most sensitive indicator of failing thyroid function. It is used as a sensitive screening test with an FT4 test to rule out hypothyroidism.

Patients with subclinical hypothyroidism typically present with minimal or nonspecific symptoms such as fatigue, depression, and hyperlipidemia. Subclinical hypothyroidism occurs most often in people aged 65 years and older. It is often transient, and TSH levels normalize spontaneously in about 35% of cases.

In hypothyroidism, respiratory control mechanisms and fluid homeostasis are dysfunctional. Ventilatory responses to hypoxia and hypercapnia are depressed. Fluid retention may lead to hyponatremia, and severe hyponatremia (<130 mEq/L) should be treated prior to elective surgery. For patients in myxedema coma presenting for emergency surgery, liothyronine (T3 hormone) can be administered intravenously. Patients should be monitored for myocardial ischemia; cardiorespiratory supportive therapy should be continued.

Anecdotally, recovery from sedation is reported to be prolonged in patients with clinical hypothyroidism. Temperature regulation is also affected, and temperature should be monitored closely. These patients have a higher incidence of myasthenia gravis, so a peripheral nerve stimulator should be used to guide muscle relaxation.