If you are feeling more fatigued, cold, losing hair and having a hard time losing weight, you may be aging, or you have an underlying thyroid disorder . An underactive thyroid is a common occurrence in modern society, and over 5 percent of the population may have a low level of thyroid hormone in blood . Once diagnosed, patients with Hashimoto's thyroiditis will be prescribed some thyroid supplement to boost their levels, generally in the form of Levothyroxine . This medication will help boost your metabolism and re-energize you after a long day of work, but certain foods may also influence the effectiveness of this drug. Make sure you are following a proper Hypothyroidism diet to get the most out of your therapy.
Foods to Improve Your Thyroid Function
When you are attempting to convert to a hypothyroidism diet, there are certain foods that you can consume to improve your thyroid's function. The first dietary changes that you should implement include cruciferous vegetables. These foods include vegetables like:
- bok choy,
- Brussel sprouts,
- collard greens,
- and cauliflower.
There is some controversy about the use of these products because of their high fiber content, but if you take your medication as directed before eating breakfast, your body will completely absorb the thyroid hormone by the time you eat these types of foods for lunch or dinner.
Another group of foods to consider when building your hypothyroidism diet includes foods that are high in selenium. One easy-to-purchase type of food would be the Brazilian nut. This nut has a high level of selenium and patients who ate Brazilian nuts were found to have elevated levels of selenium in their blood even 6 hours after consumption, and they also had the added benefit of improving cholesterol levels .
The final group of food that I think is worth mentioning when it comes to your thyroid function would be a group that has been deemed dangerous for your thyroid for decades with no scientific support for these findings: soy products. One study put this urban legend to the test and gave patients with underactive thyroids soy-based products to see the effect on their thyroid function. At the conclusion of the study, there was no adverse effect on thyroid levels. 
Foods to Avoid With an Underactive Thyroid
Stomach acid is too intense for the thyroid hormone that you are currently taking so make sure you take your dose first thing in the morning to allow your body time to absorb the sensitive hormone .
The reason for this is because both coffee and green tea have a high level of catechin, which is a compound that science has linked with reducing the level of active thyroid hormone in circulation as well as decreasing the level of thyroid peroxidase (TPO), the enzyme responsible for activating the thyroid hormone. 
When your doctor ultimately diagnoses you with thyroid dysfunction, the natural reaction is to alter your diet and make sure you limit carbohydrates in your diet to help you lose weight.
Although the rationale behind this logic makes sense medically, unfortunately, our thyroid responds in a completely different manner. A popular diet that patients striving for weight loss most often follow is the ketogenic diet; a diet high in fats but low in carbohydrates . In one study conducted to determine the effect of a ketogenic diet on the long-term health of patients who have epilepsy, the study found that out of the 120 participants, 20 of them developed hypothyroidism because of their diet choice .
The principle behind the ketogenic diet is designed around the basis of tricking the body into thinking it's starving. During periods of starvation, our metabolisms naturally are reduced to make sure we save the limited energy we have left. Deciding to starve yourself is essentially deciding to switch off your thyroid gland. If you do this fasting only for a limited period, there will be changes to your thyroid stimulating hormone (TSH) levels, but there will not be a noticeable difference in your active thyroid hormones.
In one study, scientists monitored religious fasting during Ramadan, and they observed the effects on the thyroid gland. Even after a month of irregular diets, candidates were found to have an altered level of hormones, and as you can see from the previous study about Ketogenic diet if patients continued this practice, thyroid dysfunction was sure to follow. 
Too much food can also exacerbate an underactive thyroid. Therefore, it is important that you consult with a dietitian after being diagnosed with an under performing thyroid to optimize a hypothyroidism diet that is designed especially for you .