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In this case the tremors or trembling could be due to electrolyte disturbance. Taking soups, broths, lemonade, oral rehydration solution etc may help.
Tremors are often associated with drugs, alcohol, smoking, lack of sleep, poor sleep patterns, stress and fatigue. They can be seen in peripheral neuropathies of diabetes, in hyperthyroidism, parasomnia, multiple sclerosis, TIAs or transient ischemic attacks or mini strokes, hypoglycemia, disorders of calcium metabolism, hyperparathyroidism etc. Certain epilepsies too could be the cause. Consult an internal medicine specialist or your PCP. Discuss these possibilities with your doctor. A comprehensive investigation is required keeping all the points in mind. You will need blood sugar monitoring, MRI of brain and cervical spine, T3, T4, TSH, sleep studies, serum electrolytes, brain EEG, etc to reach a diagnosis.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your PCP for primary examination followed by proper referral. Take care!
First of the temperature of the body is controlled by the hypothalamus. It acts as a thermostat trying to balance the perfect temperature throughout the body's chemistry. First thing suspected would be an infection somewhere. But you've mentioned tests came back norm.My other questions are what are if any medications he has been taking. Any dehydration, added stress to the heart, arthritis etc?