About 4.6% of the U.S. population ages 12 and older has hypothyroidism. Hypothyroidism is more common in women and in people older than age 60. There is a range of hypothyroidism from subclinical hypothyroidism to myxedema coma. Usually, therapy for hypothyroidism remains fairly stable unless there is a change in body weight or interaction with another medication.
Thyroid gland dysfunction can be due to:
- Hashimoto’s disease, an autoimmune process in which lymphocytic infiltration and fibrous tissue accumulation cause replacement of normal thyroid tissue
- Congenital hypothyroidism
- Surgical removal of part or all of the thyroid
- Radiation treatment of the thyroid
- Some medicines such as amiodarone and iodine (Wolff-Chaikoff effect)
- Central hypothyroidism: insufficient production of bioactive TSH due to pituitary or hypothalamic tumors, inflammatory (lymphocytic or granulomatous hypophysitis) or infiltrative diseases, hemorrhagic necrosis (Sheehan’s syndrome), or surgical and radiation treatment for pituitary or hypothalamic disease.
- Hereditary disorders of the iodothyronine synthesis pathway (thyroxine [T4] and triiodothyronine [T3])
Hypothyroidism is also more common in people with a history of:
- Prior thyroid disease such as a goiter
- Prior thyroid surgery
- Receiving radiation treatment to the thyroid, neck, or chest
- Family history of thyroid disease
- Recent pregnancy (within the past 6 months)
- Chromosomal disorders such as Down’s or Turner’s syndrome
- Other auto-immune conditions such as: type 1 diabetes, Addison’s disease, Sjögren’s syndrome, pernicious anemia, rheumatoid arthritis, lupus
- Receiving therapy with drugs such as lithium, interferon alpha, amiodarone, or excess iodine ingestion such as kelp
Symptoms and signs of hypothyroidism:
- weight gain
- a puffy face
- cold intolerance
- joint and muscle pain
- dry skin
- dry, thinning hair
- decreased sweating
- heavy or irregular menstrual periods
- fertility problems
- slowed heart rate
- delayed Achilles reflex time
There are very few studies about the impact of hypothyroidism on job productivity (presenteeism and absenteeism). A Danish study examined records for people with hypothyroidism and found that while the risk of taking sick leave was not significantly affected, people with hypothyroidism faced longer recovery time than euthyroid peers if they had to take sick leave in the first year after diagnosis. There was no significant effect upon workplace absenteeism after the first year, post-diagnosis. Another study showed that night shift workers may have an increased risk of developing subclinical hypothyroidism.
A comprehensive history and physical examination must be done and focus on possible signs and symptoms of hypothyroidism and its impact on various systems. A workability questionnaire on thyroid disease can be found at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4483246/.
The LEO evaluation in people with hypothyroidism needs to focus on possible co-morbidities which improve with appropriate therapy of the underlying thyroid condition.
- Neuromuscular symptoms present in 30-80% of patients with hypothyroidism.
- Patients may complain of muscle cramping, proximal symmetrical muscle weakness, muscle stiffness, and exercise intolerance. These symptoms improve or disappear with correction of the hypothyroidism.
- Slowed muscle relaxation and contraction are noted in hypothyroid myopathy.
- Hypothyroid myopathy can be a polymyositis-like myopathy with proximal muscle weakness and an increased creatine kinase level. However, it sometimes manifests as muscle enlargement (pseudohypertrophy); in adults, this condition is called Hoffman syndrome.
- Rhabdomyolysis may be associated with hypothyroidism, especially with dehydration. Rarely, myopathy may be the sole presenting manifestation of hypothyroidism.
- Certain drugs, such as lipid-lowering medications such as statins, may exacerbate myopathy in patients with hypothyroidism, especially with dehydration. (Note: hypothyroidism may also be a cause for increased cholesterol.)
- Dupuytren’s contracture, limited joint mobility and carpal tunnel syndrome were commonest in hypothyroid patients. Deep tendon reflexes are delayed in approximately 85% of patients with hypothyroidism.
- The creatine kinase level can be very high (10-100 times greater than the normal level) in some patients, but it has no correlation with weakness. The level returns to normal with adequate treatment.
- The myopathy improves within 2-3 weeks, but months may be required for it to resolve completely.
- For LEOs, no restrictions are needed if there are no signs or symptoms of muscle weakness, muscle pain or Dupuytren’s contracture.
- Short-term restrictions will depend on the specific physical demands of their job. If there are significant physical demands, short term restriction may be necessary until the LEO is euthyroid. This may take up to 6-8 weeks, but improvement may be apparent as early as 4 weeks post therapy. The LEO physician may use training activities to evaluate the LEO if there are unresolved questions on the LEO’s ability to perform strenuous essential job functions.
Cardiovascular System (CV)
Hypothyroidism may be associated with the following CV conditions which usually resolve with treatment of hypothyroidism:
- Cardiac arrhythmias such as sinus bradycardia.
- Impaired cardiac contractility and diastolic function
- Increased systemic vascular resistance
- Increased serum cholesterol
- Increased C-reactive protein
- Increased homocysteine
- Decreased cardiac contractibility
- Congestive heart failure
Assess for signs and symptoms of cardiovascular conditions above.
- If there are signs and symptoms of cardiovascular conditions, restrictions should be consistent with the LEO guidance in the CVD chapter below. Generally, the cardiovascular conditions resolve once the hypothyroidism is treated.
- If there are no signs or symptoms of cardiovascular manifestations of hypothyroidism, no restrictions in duty are necessary.
- Sinus Node Disturbances: Sinus arrhythmia such as bradycardia is not generally associated with symptoms or known to degenerate into malignant arrhythmias. Asymptomatic LEOs with sinus node disturbances and no structural heart disease should not be given job restrictions.
- Heart Failure: Heart failure associated with hypothyroidism usually resolves when the person is euthyroid. If the LEO has symptoms of heart failure, restrictions should be given since heart failure also affects the ability of the LEO to perform all critical job functions due to reductions in aerobic capacity (NYHA Class I has an aerobic capacity of about 6-7 METs). The criteria for unrestricted duty in the presence of heart failure remains the previous cited value of 12 METs obtained in a routine Bruce protocol EST. The LEO should be re-evaluated after the hypothyroidism has been treated usually in 4-6 weeks.
Hypothyroidism may be associated with the following neuro-psychiatric conditions which usually resolve once the hypothyroidism is treated: (Note.
If symptoms persist when the person is euthyroid, other causes of the symptoms should be evaluated.)
- Mild cognitive dysfunction, especially in memory functions
- Alterations in mood
LEO Evaluation (see Mental Health chapter)
- Assess for signs and symptoms of conditions above.
- A Patient Health Questionnaire (PHQ9) may be helpful to identify depression https://www.integration.samhsa.gov/images/res/PHQ%20-%20Questions.pdf.
- A formal psychological evaluation may be necessary to determine if the LEO has met DSM-5 criteria for a specific condition such as depression.
- If there are significant signs of depression, a formal psychological evaluation may be necessary and short-term restrictions may be necessary. Re-evaluate when hypothyroidism has resolved.
- If cognitive function such as mild memory loss is found, short term restrictions such as “no work requiring full attentiveness” may be necessary until the hypothyroidism is treated.
- Elevated TSH (thyroid stimulating hormone)
- Low free thyroxine (T4)
- There is a range from subclinical hypothyroidism elevated TSH) and normal free T4 to overt hypothyroidism (elevated TSH and low free T4)
- Elevated total cholesterol and low-density lipoprotein (LDL)
- Increased creatine kinase due to an increase in the MM fraction, which can be marked and may lead to an increase in the MB fraction. There is a less marked increase in myoglobin and no change in troponin levels even in the presence of an increased MB fraction.
- Hyponatremia often is seen in patients with hypothyroidism.
- Usual therapy is oral L-thyroxine.
- The daily dosage of L-thyroxine is dependent on age, sex, and body size, co-morbidities (such as coronary heart disease or prior history of thyroid cancer), and cause of hypothyroidism. For example, some people may have some residual thyroid function post thyroid surgery or radiation.
- Ideal body weight is best used for clinical dose calculations because lean body mass is the best predictor of daily requirements.
- With little residual thyroid function, replacement therapy requires approximately 1.6 μg/kg of L-thyroxine daily
- Dose adjustments are guided by serum TSH determinations 4-8 weeks following initiation of therapy, dosage adjustments, or change in the L-thyroxine preparation. While TSH levels may decline within a month of initiating therapy with doses of L-thyroxine such as 50 or 75 μg, making adjustments with smaller doses may require 8 weeks or longer before TSH levels begin to plateau.
- Once the TSH levels are in the normal range, the TSH can be tested on a yearly basis unless there are changes in body weight or interactions with other medications.