12/08/2025
MAC in hypothyroidism and hyperthyroidism
● Hypothyroidism
MAC Value: Unchanged. The partial pressure of the anaesthetic gas required at the brain to prevent movement in response to a surgical stimulus is not directly affected by the level of thyroid hormone.
Clinical Reality: Patients with hypothyroidism are clinically very sensitive to the effects of all anaesthetic agents, including volatile anaesthetics. They will appear to require less anaesthetic.
Explanation: The sensitivity is not due to a change in brain requirement (MAC) but rather due to changes in drug delivery and systemic effects (pharmacokinetics).
→ Reduced Cardiac Output: A lower cardiac output means that the anaesthetic gas taken up from the lungs is delivered more slowly to the rest of the body's tissues. This causes the partial pressure in the blood and brain to rise more quickly toward the inspired concentration, leading to a faster induction of anaesthesia.
→ Altered Systemic Effects: Hypothyroid patients often have blunted baroreceptor reflexes and are prone to significant hypotension and bradycardia with anaesthetic agents.
→ Associated Hypothermia: Severe hypothyroidism can be associated with hypothermia. Hypothermia itself is a potent cause of decreased MAC. So, if a hypothyroid patient is cold, they will require less anaesthetic, but the cause is the low temperature, not the thyroid state directly.
🛑 Clinical Approach: You do not change your target MAC on the vaporizer based on thyroid status. However, you must be extremely cautious, titrate the agent very slowly, and be prepared to manage significant haemodynamic instability.
● Hyperthyroidism
MAC Value: Unchanged. Similar to hypothyroidism, the intrinsic requirement of the brain for the anaesthetic is not altered by excess thyroid hormone.
Clinical Reality: Anaesthetising a patient with untreated or poorly controlled hyperthyroidism can be very challenging. They have a hypermetabolic, hyperdynamic state.
Explanation:
→ Increased Cardiac Output: A high cardiac output means anaesthetic gas is rapidly taken up from the lungs and distributed throughout the body. This can slow the rate of rise of the partial pressure in the brain, potentially leading to a slower induction of anaesthesia.
→ Hyperdynamic State: The main challenge is managing the tachycardia, hypertension, and risk of arrhythmias caused by the high sympathetic tone. The focus is on controlling this sympathetic output with beta-blockers and ensuring adequate anaesthetic depth to blunt surgical stimulation, not on simply increasing the MAC value.
→ Associated Hyperthermia: Hyperthyroidism can cause a mild increase in core body temperature. Hyperthermia is a cause of increased MAC. If the patient is hyperthermic, they will require more anaesthetic, but again, this is due to the temperature change itself.
🛑 Clinical Approach: The primary goal is to ensure the patient is euthyroid (in a normal thyroid state) before any elective surgery. If emergency surgery is required, the focus is on aggressively managing the hemodynamic effects with beta-blockers and ensuring deep anesthesia to prevent a catastrophic thyroid storm, rather than focusing on a specific MAC value.
