Wordpress get_template_directory_uri() returns http instead of https

Check the $_SERVER['HTTPS'] value. This should be set to on or 1. If it has any other value while being set, this function will output http rather than https.

See: https://core.trac.wordpress.org/browser/tags/4.5.3/src/wp-includes/functions.php#L4025

Going by this comment the initiator added:

in my case we were using Load Balancer server and the SSL certificate was install on load balancer

The answer from @pbond scratches the surface to the root cause of the issue. The WordPress is_ssl() function checks the $_SERVER['HTTPS'] and $_SERVER['SERVER_PORT'] to check if the current page is being accessed via https but the load balancer is most likely requesting your content on the non-SSL port 80.

One good fix for this is to use the X-Forwareded-Proto HTTP header to figure out which protocol the client is actually using on the other side of the Load Balancer.

With Apache 2.2, you could add this to your configuration:

<IfModule mod_setenvif.c>  SetEnvIf X-Forwarded-Proto "^https$" HTTPS</IfModule>

Another possible fix (alluded to by @Roberto Poblete but not explained) is to add this to wp-config.php

if (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')$_SERVER['HTTPS'] = 'on';

I have this to thank for sending me in the right direct

view notes of link https://codex.wordpress.org/Function_Reference/is_ssl

if (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')$_SERVER['HTTPS'] = 'on';